I reject both arguments, but here I will address the biblical commands in question, and Copan's interpretation – leaving aside the question of whether Martin makes a good case based on said commands.
22:23 If there be a young lady who is a virgin pledged to be married to a husband, and a man find her in the city, and lie with her; 22:24 then you shall bring them both out to the gate of that city, and you shall stone them to death with stones; the lady, because she didn't cry, being in the city; and the man, because he has humbled his neighbor's wife: so you shall put away the evil from the midst of you. 22:25 But if the man find the lady who is pledged to be married in the field, and the man force her, and lie with her; then the man only who lay with her shall die: 22:26 but to the lady you shall do nothing; there is in the lady no sin worthy of death: for as when a man rises against his neighbor, and kills him, even so is this matter; 22:27 for he found her in the field, the pledged to be married lady cried, and there was none to save her. 22:28 If a man find a lady who is a virgin, who is not pledged to be married, and lay hold on her, and lie with her, and they be found; 22:29 then the man who lay with her shall give to the lady's father fifty shekels of silver, and she shall be his wife, because he has humbled her; he may not put her away all his days.
In his analysis of the passage – and his reply to Martin -, Copan distinguishes the following cases:
Case a. Consensual sex between a man and a “betrothed girl”.
Copan claims that the act is tantamount to adultery. However, that may or may not be the case. It may well be that the woman was “pledged” to a man against her will , and then chose to have sex with someone else. Clearly, she would not be at fault in that case. But if she does that, then she and the man she has sex with are stoned to death, if the biblical command is followed. That is of course, morally abhorrent. On the other hand, there is no punishment for the man who would sell her to a rapist, or to the rapist.
But moreover, even if the command were applicable only to cases in which she had agreed to the engagement – but that's not the case -, it is not the case that she or the man she has sex with consensually deserve to be stoned to death. Nor was it morally acceptable for anyone to follow that command willingly, and stone them to death. Frankly, even in that case, the command is morally atrocious, and the Bible is also promoting false moral beliefs – in this case, the belief that she deserved to be stoned to death.
A morally perfect being with the power to stop it would never command anything remotely like that.
Case b. Rape of an “engaged girl”.
In this case, only the rapist is punished by death, unless she failed to cry, in which case she is also stoned to death. In context, it’s apparent that there is an assumption that because she did not cry, she consented to having sex with the man in question. However, it is also apparent that even if she did not cry, she may well not have consented to having sex with him. Maybe the man raped her and (for instance) had a knife on her throat, or maybe she was terrified, she froze, etc. There are a number of possibilities. Copan, however, claims that “because she did not call for help, it can be assumed that she consented to the man's advances”. In other words, he considers the fact that she did not cry to be conclusive evidence to convict her of adultery, and enough to morally justify stoning her to death. As before, this is appalling.
Incidentally, even though the rapist is to be stoned to death in this case, he's being punished not for rape (which also would be an improper punishment, but that aside), but for having sex with a woman pledged to another man. This is apparent from the facts that:
1. He is stoned to death regardless of whether the sex with the “pledged” woman was consensual, as indicated in a. above.
2. If the woman is not pledged to another man, he is only fined even if he rapes her, as explained below.
Case c. Rape of an “unengaged/single girl”.
In this case, the rapist is only forced to pay a fine, and then he is forced to marry his victim, so he can continue to rape her legally, and with no further punishment, at least if her father agrees.
Moreover, if she were to escape from his husband/rapist and have sex consensually with another man, she would be stoned to death for adultery. The biblical dispositions on the matter are not just morally bad, but appalling – as are so many other biblical commands, as I have pointed out elsewhere. Moreover, the Bible implies that she deserves that punishment in a case like that, so it promotes absurdly false moral beliefs.
What does Copan say about that?
He claims that the case under consideration is one in which a man takes advantage of a minor with her consent, and the Bible does not give the man or the father the choice to refuse, allegedly to protect her, because she gets security and provision – allegedly.
However, the biblical passage is not limited to cases in which she consented – never mind she may not have been mature enough to consent, or at least to consent in a way that would make sex with her morally acceptable.
This is clear both from the text, and the fact that there is no other disposition establishing a separate punishment for someone who rapes a woman who is single and not pledged.
Moreover, even if one limits this biblical command to cases in which the minor was not forced – but it was not so limited case -, she may be forced later; she has no choice to reject the marriage. And when she's married, her husband is legally entitled to rape her. So, a man takes advantage of a minor, and then she's forced to marry him, where she gets “security and provision” against her will by a man who can legally rape her at will. The biblical disposition remains atrocious.
31:1 Yahweh spoke to Moses, saying, 31:2 Avenge the children of Israel of the Midianites: afterward you shall be gathered to your people. 31:3 Moses spoke to the people, saying, Arm you men from among you for the war, that they may go against Midian, to execute Yahweh's vengeance on Midian. 31:4 Of every tribe one thousand, throughout all the tribes of Israel, you shall send to the war. 31:5 So there were delivered, out of the thousands of Israel, a thousand of every tribe, twelve thousand armed for war. 31:6 Moses sent them, one thousand of every tribe, to the war, them and Phinehas the son of Eleazar the priest, to the war, with the vessels of the sanctuary and the trumpets for the alarm in his hand. 31:7 They warred against Midian, as Yahweh commanded Moses; and they killed every male. 31:8 They killed the kings of Midian with the rest of their slain: Evi, and Rekem, and Zur, and Hur, and Reba, the five kings of Midian: Balaam also the son of Beor they killed with the sword. 31:9 The children of Israel took captive the women of Midian and their little ones; and all their livestock, and all their flocks, and all their goods, they took for a prey. 31:10 All their cities in the places in which they lived, and all their encampments, they burnt with fire. 31:11 They took all the spoil, and all the prey, both of man and of animal. 31:12 They brought the captives, and the prey, and the spoil, to Moses, and to Eleazar the priest, and to the congregation of the children of Israel, to the camp at the plains of Moab, which are by the Jordan at Jericho.
So, those ancient Israelite troops attacked and killed every single man, apparently regardless of whether those men were combatants or not, whether they were too ill or old to fight, or whether they had surrendered. They also took the women and their children as spoils of war. In other words, they took them as slaves as slaves, and brought them to Moses and to one of Yahweh’s priests.
The soldiers surely would expect to take women as sex slaves – their consent was not required, as usual -, raping them repeatedly – in accordance to Yahweh’s laws.
What was Moses’s reaction?
31:13 Moses, and Eleazar the priest, and all the princes of the congregation, went forth to meet them outside of the camp. 31:14 Moses was angry with the officers of the army, the captains of thousands and the captains of hundreds, who came from the service of the war. 31:15 Moses said to them, Have you saved all the women alive? 31:16 Behold, these caused the children of Israel, through the counsel of Balaam, to commit trespass against Yahweh in the matter of Peor, and so the plague was among the congregation of Yahweh. 31:17 Now therefore kill every male among the little ones, and kill every woman who has known man by lying with him. 31:18 But all the girls, who have not known man by lying with him, keep alive for yourselves.
So, Moses was not angry because his troops wanted to enslave children – both boys and girls -, or because they wanted to enslave adult women, in general.
Instead, Moses was angry because the soldiers had kept the women who were not virgins alive, and apparently – unless he was lying about his motivation, but why would he? - he believed that all of the women who were not virgin had “caused” the children of Israel to trespass against Yahweh. Furthermore, he believed apparently that those women deserved to be killed for that.
So, Moses commanded that all women who were not virgins be killed. He further commanded that every male child be killed as well.
On the other hand, Moses told his men to keep the virgin girls for themselves - implicitly for sex slavery and repeated rape in the future - if they so choose.
It is not specified what he wanted his men to do with adult women who were virgins – if there were any -, or with girls who were not virgins because they had been raped already. Given context, it seems all non-virgins were to be killed regardless of whether they were adults. As for adult women, also context indicates that they were presumed to be non-virgins – which might or might have been true, but of course does not justify the killings in any way -, and were to be executed. In any event, his command was immoral regardless of how he split the girls and the women between those that were to be killed and those that were to be enslaved and raped whenever their masters chose.
So, in particular, Moses:
1. Falsely blamed all non-virgin women for a plague. Of course, regardless of what some Israelite men did in order to have sex with at most some – surely not all – of those women, those men made their own choices, and in any case, surely blaming those women for a plague was absurd.
2. Murdered all of the boys but kept girls for slavery, including sex slavery and thus repeated rape.
Given that behavior, even independently of other events, one should reckon – assuming the account is accurate - that Moses was a moral monster, a mass-murderer and mass-rapist – regardless of whether he engaged in mass rape personally, he surely was responsible given his commands.
So, what was Yahweh's reaction?
31:25 Yahweh spoke to Moses, saying, 31:26 Take the sum of the prey that was taken, both of man and of animal, you, and Eleazar the priest, and the heads of the fathers' houses of the congregation; 31:27 and divide the prey into two parts: between the men skilled in war, who went out to battle, and all the congregation. 31:28 Levy a tribute to Yahweh of the men of war who went out to battle: one soul of five hundred, both of the persons, and of the cattle, and of the donkeys, and of the flocks: 31:29 take it of their half, and give it to Eleazar the priest, for Yahweh's wave offering. 31:30 Of the children of Israel's half, you shall take one drawn out of every fifty, of the persons, of the cattle, of the donkeys, and of the flocks, even of all the livestock, and give them to the Levites, who perform the duty of the tent of Yahweh
Yahweh did not punish Moses or his men, or expressed any disapproval whatsoever, but proceeded to give instructions as to how to divide the spoils of war, including the people taken as slaves.
Even assuming for the sake of the argument that there was no human sacrifice and Yahweh’s “wave offering” was “just” giving some captives to the priests as slaves, the fact remains that Yahweh instructed Moses – and, through him, many other ancient Israelite men – to engage in mass slavery and mass rape. Yahweh is another moral monster, like Moses.
So, what happened then?
They just proceeded to divide the spoils, after murdering the captive women and perhaps girls who were not virgins – or who they believed were not virgins, anyway – and the children of those who had children. While it would have been monstrous to do that to a single woman or to a single child, one may point out here that the massacre was massive, as one may reckon from the description - 32000 girls and/or women were taken as slaves.
So, what does Copan have to say about these matters?
First, he says the command to kill all of the boys and non-virgin women may “seem harsh”, but is justified because the young girls not to be killed had not “debased themselves in the orgiastic worship of Baal”.
But Copan's claim is clearly false, because:
1. Even if many of those being killed were having orgies, that's of course no good reason to kill them. And if a powerful entity claims it is and commands that they be killed, people ought not to believe that such entity is morally good, let alone perfect.
2. All of the boys were to be killed as well. Had they “debased themselves” too?
This is, again, appalling. Of course, even if the ancient Israelites in question had witnessed that a powerful entity gave such command, they should not have followed it willingly.
Second, Copan claims that in those cases it was permissible not to rape, but to “take a wife”, so that they would be allegedly protected against slavery. But that simply makes no sense – i. e., it's absurdly improbable as an interpretation of the text.
Let's consider the situation for a moment: those women had just witnessed how their mothers, sisters, brothers, etc., had been slaughtered – including non-combatants, of course -, and then, they were taken captives.
The Bible clearly indicates how the attackers were dividing those women among the victors, and how their leaders gave instructions to do so. It's obvious that many of those women would have rejected being taken by those men, but – again – there is no indication at all that they were allowed to walk free and take their chances if they so chose – no, the text is clear: they were forced, the choice was only made by the attackers.
Also, there is no indication whatsoever that consent was required by law before they could be taken as “wives” - and husbands were allowed to rape their wives in that society, by the way.
The fact is that the Bible describes a clear-cut case of mass murder, mass slavery, and mass rape. And it's one of many cases in which the Bible describes moral atrocities and yet claims those actions were morally acceptable, good, or even obligatory.
A link can be found in Copan's website.
 Jeffery Jay Lowder cites Copan's paper and replies to one of Copan's metaethical arguments in a post at the Secular Outpost.
 I wrote a much longer moral case against Christianity, which can be found here.