I differ from
@The Revolting Man on this in that I don't consider them automatically married from having sex, however I consider him to have an obligation to take her as a wife. If he doesn't do this, he is sinning by not fulfilling his obligation.
If she is not a virgin, he is not obliged to marry her. This is because if a woman was obliged to marry every man she slept with, she would be obliged to be polyandrous, which is sinful. Imagine a woman who sleeps with one man, hides it, sleeps with another, and then it all comes out and she is now required to marry both. Doesn't work. That's why this only applies to virgins. So if she's a non-virgin, there is no
obligation to marry her, however if she's available it would obviously be the right thing to do.