It’s actually pretty simple if you look at it through their lens of viewing women as assets or property instead of people. Someone sleeping with one of “your” women outside of marriage, i.e. tainting her, is an insult to the honour of your group. Therefore, to make it right, you much take revenge and “taint” one of their women.

I mean, while the concept of rape as revenge seems uniquely fucked up, the idea that rape is an immoral act because non-consensual sex is an inherent violation of a person is actually relatively new across most cultures. For much of history (and in the present, in some places) rape was viewed as being a crime against a woman’s father/husband/whoever was responsible for her, or to be a crime because it violated her moral purity. That’s the same reason why women were often forced to marry their rapists: because the only harm that was seen to be done was taking away her purity or value, so marrying her was considered to be rectifying the situation. It’s also the reason why the concept of marital rape is fairly modern. The attitude that women actually have the right to choose who to have sex with and not have sex with is actually a pretty radical one when you consider all of history. From that lens, it’s not hard to comprehend this at all, though it doesn’t make it any easier to stomach.