Downplayed: Bob has a relationship with Alice that does include sex, but only after marriage.
Justified: The story takes place In a World where a Daddy DNA Test isn't readily available to determine who a baby's father is (and birth control doesn't really exist). It could also be a setting in which the groom and/or his family shells out large sums of money/cattle/land/whatever else is used as currency to acquire a bride.
Defied: "Guys who measure a woman's worth on her hymen are just insecure about their performance in bed."
Discussed: "Betty is fun and sexy, but would you really want her to be the mother of your kids?"
Conversed: "There is more to a woman than her sexuality; get to know her, and then decide whether to commit or not."
Bob chooses Alice, the "good" girl, but she doesn't have a heck of a lot in common with him; he chooses her on the basis that "she'll make a good wife and mother." Eventually, Bob becomes bored with Alice, and finds himself tempted by Betty and her ilk. Alice is hurt because Bob is losing interest in her and neglecting her, and Bob is hurt because he now feels like it was a big mistake to marry Alice, he feels guilty for cheating with (or just lusting after) Betty, he blames Betty for his problems, Alice is jealous of Betty, and (ironically) Alice and Bob 's home life is in more danger than it likely would have been if he had just chosen Betty in the first place. The marriage becomes troubled, and might just end altogether depending on the setting.
Alice and Bob are Cute But Psycho. Betty is The Messiah. While Alice and Bob become more and more sociopathic Stepford Smilers who want to live the perfect, suitable suburban life, Betty is a healthy, generous, and romantic person. She ends up finding someone who loves her and thinks that she is important for what she is inside, has the epitome of a beautiful love and family life, and Bob and Alice's children end up asking her help, because they do not want to repeat the life of their parents.
Reconstructed: Sex Is Good; it is realized that the "good" girl Bob marries likely wants to have (and enjoy) sex, not just Lie Back and Think of England, and the "wild" woman Bob hooks up with is (barring a drug addiction or other major malfunction not related to her sexuality) very likely capable of being nurturing and faithful.
Played For Laughs: Alice may be a nice girl, but she's a total Gonk.