Basic Trope: A woman who loses her virginity (whether it was her choice or not) or gets raped (whether she was a virgin or not) is considered finished and no longer desirable. Related to My Girl Is Not a Slut.
Alice is raped and Bob either breaks up with her, or rethinks his plans to ask her out, propose to her, etc.
Alice loses her virginity and Bob is no longer interested in her.
Alice is raped, and Bob is suddenly reluctant to ask her out or propose- but it's because he doesn't want to make her uncomfortable by pressing a relationship on her while she's still traumatized.
Until people start talking about Alice's "promiscuity", laughing at Bob for accepting "damaged goods," etc. Then Bob breaks off the engagement.
Alternately, the "rapist" was Bob, playing a little joke before the marriage.
Alice's skirt is blown up, and she runs away in shame, shouting "I can no longer be a bride!"
Alice's trousers fall down her legs, and she is seen by Charlie. Alice cries about how she can no longer be a bride, when Charlie simply thinks she needs a tighter belt.
Alice is raped just days before her wedding. Bob still loves Alice, and still decides to go through with the marriage. Then people start talking, and Bob breaks up with Alice. Then he feels bad about it, and apologizes to Alice and the wedding is on again.
Bob has absolutely no intention of breaking up with Alice but is confused about how to help her. Alice mistakes that for lack of love and breaks up. Bob pleads with Alice and Alice realizes he really does love her. Together they work through it. Ah....
Alice's virginity is not a factor in whether or not Bob loves or marries her.
No commentary is made on it.
Alice loses her virginity to Bob.
The author is from a time or place where this is the norm.
Sure, Bob dumped her, and her parents disowned her, but a more enlightened person comes along and gladly accepts Alice, not caring what others think. Alice gets a second chance.
Alternately, after Alice is abandoned by Bob and her family because she's no longer "pure," she is forced to live independently, relying only on herself for support and becoming a strong, self-sufficient woman in the process.