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Women are literally divided into a caste system in which they can either be married or prostitutes
Bob has a relationship with Alice that does include sex, but only after marriage.
Bob wants a girl who's over the party scene, feeling that a Hard-Drinking Party Girl who Really Gets Around is too immature with whom to settle down, or won't be a good role model to any children they might have.
Justified: Bob, Alice and Betty live in a society that closely links open sexuality with morality: Openly sexual = "whore", modest or non-sexual = "respectable woman".
Defied: After a series of critical thinking, Bob decides to see each girl as more than whether they're virgins.
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."
Implied: They are characters in a kids show, so the writers keep it family friendly and avoid naming sex directly. Instead, Bob does not want to date Betty because "she's had too many boyfriends".
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 an All Loving Heroine. 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.