This is an old post, but I ll respond since it is still here. As far as Old Testament law goes, much of it is law for Israel. There are passages that explain this. Israel was to be distinct from other nations, and what we read in the Pentateuch is a law given to them. It doesn t say that Gentiles can t eat pork. Noah and his descendants were given animals, including pigs, to eat. So Hebrews were forbidden to eat pork, but not Gentiles. The Torah was given to Israel. It was written to them, but also for us.
Back then, girls married young. There was no going away to college. People primarily worked on farms. And it is implied and indicated that fathers gave their daughters in marriage and were responsible for authorizing a marriage. If a man raped a betrothed girl, it was treated like adultery. He was stoned. If he raped or seduced an unbetrothed girl, if her father would let them, he was required to marry and never divorce her. If not, he still had to pay her bride price.
A girl who was not a virgin who allowed herself to be married off as if she were one, could face the death penalty, but there was no death penalty for being raped.
So consider different situations. Some sicko seduces or rapes an unbetrothed girl. The father collects the bride price from him, but doesn t let him marry her. Now consider another two young people. The boy and girl make eyes at each other. The girl says she is going to milk to goat. She goes out to the tree where the goat is tied up and meets the boy in the village she likes. They start touching and kissing. He gets too aggressive with her, kissing her, and he rapes her.
Why might a father let this man marry her? Consider what the Old Testament says about marriage, two shall be one flesh. If a woman sleeps with another man before marriage, she violates that. Even if there is a rape, it goes against that principal (and of course the rapist caused it to happen.) There was some research from 1990 by Teachman that showed that women who had slept with multiple men before marriage were much more likely to get a divorce than virgins and those who had only slept with the men they ended up marrying. I believe if a woman sleeps with only one man, she bonds with him. It s the idea of one man marrying one woman for life. Now, men and women get widowed, too. And God is able to restore those who sin, and heal the emotional wounds of those who are raped as well.
Does a Christian woman have to marry her rapist? Well there are other issues like monogamy and not being unequally yoked to an unbeliever. most of us aren t Jewish, and so realizing that all those laws weren t commanded to everyone does help. But I hope that my post helps you understand why the marrying the rapist thing was in there. The obligation is on the man to marry the girl, but the girl s father can reject a man to marry his daughter.