i) Atheists like to quote Deut 21:10-14 as a case of Scripture sanctioning rape or sex slavery. I've discussed this before. The passages makes provision for war brides, not sex slaves.
ii) In addition, it's fallacious to infer that a law code condones whatever it regulates. For instance, a libertarian legislator might propose a law to decriminalize possession of Marijuana, not because he approves of potheads, but because he thinks the "war on drugs" is more detrimental than letting people smoke pot.
iii) The contention that this is rape or sex slavery is based on the fact that it's a forced marriage. However, one problem with that objection is that it disregards the circumstances in which this issue crops up. The setting involves a warrior culture in which the able-bodied men were killed in combat, thereby widowing their wives. The women no longer have any men to protect them or provide for them, which is a dire situation for women in the ancient Near East.
So it's a question of how to play the hand you were dealt. We are often "forced" into situations we dislike, "forced" to make decisions we dislike, due to onerous circumstances beyond our control.
iv) However, I'd like to approach the issue from a different angle. Suppose the scenario involved war grooms rather than war brides. Suppose you have a queen. The army fights at her behest. Her army defeats the enemy. Some of the war captives are handsome men. She wants to marry one of them, and she exercises her royal prerogative to do so. The male war captive is "forced" into a marriage with the queen.
Is that rape? If they were honest, I doubt people would characterize the arrangement in those terms because they don't think men must be forced to have sex.
Or let's vary the illustration. Suppose the queen adds some of the handsome male war captives to her harem. They are her sex slaves. They are available for her pleasure.
Is that rape? The male war captives didn't choose to be harem boys to service the lascivious monarch. But even if they find the prospect distasteful, is it rape?
This poses a dilemma for atheists. Many atheists pride themselves on their egalitarian views of men and women. They champion feminism. If they think men and women should be treated alike, if they don't think a queen who has sex with a harem boy is raping him, then the war bride scenario isn't rape.
If, on the other hand, they admit that men and women are wired differently in this regard, then they must forfeit their feminism. Opt for one or the other: you can't have both!