Please describe a situation where the girl is free to not marry the rapist and at the same time the rapist "must marry the girl." Forcing the rapist to marry the girl necessarily forces the girl to marry the rapist. If she's not forced to marry him, then he's not forced to marry her.
Please provide a Biblical quote which justifies the addition of your conditional.
First, please keep in mind that I am not defending kurious_kathy in any way. I am simply pointing out that many of the most-quoted Biblical passages used in arguments against theists are often mistranslations or misinterpretions (or both). Many (maybe most) are far from cut and dry.
Case in point, here is the NIV translation of a very similar passage (Exodus 22:16) to the one in question:
16. If a man seduces a virgin who is not pledged to be married and sleeps with her, he must pay the bride-price, and she shall be his wife.
"She shall be his wife" seems pretty cut and dry, right? If the guy sleeps with her, he marries her. Period, end of story. But then look at the next passage:
17. If her father absolutely refuses to give her to him, he must still pay the bride-price for virgins.
Oh, but wait! Didn't it say in the previous passage that he
must marry her? Obviously, that's not exactly an accurate interpretation of what the passage means. Upon further inspection, it simply means that he has no choice in the matter. The passage in question in Deuteronomy is very similar, except that a rape has taken place, and in a similar manner the point is that the rapist doesn't have a choice in the matter of whether or not to marry her (and in that case, whether or not to divorce her once they are married).
Keep in mind that the quote above is from the same translation that was used for the posts to which I was responding. Often, if you look at the original Hebrew and consult different translations, you'll start to see a slightly different story emerge.
I'm actually on your side of this argument, but these sorts of quotes from the Bible are rarely as definitive as you might want to believe.
-Bri