A minister or Sunday School teacher brings it up in his/her sermon.
An abstinence-only sex-ed class
Exploited: Bob is an idealistic hero, but not above sexual urges. Claire is a Card-Carrying Villain and seduces him with her feminine wiles because having premarital sex will bring him over to The Dark Side.
Defied: Alice and Bob join a religion (yes, there are a few) that believes that Sex Is Good, or at least not evil.
Discussed: "What's so bad about sex anyway?"
Conversed: "While it's true that there exists the possibility of things going wrong, if you have sex responsibly (use condoms, get tested regularly, and limit the number of partners you have), they're less likely to. It's just people are afraid of things going wrong, and that fear for their safety and their partner's safety translates as a fear of sex and sexuality.
Sex is treated as a Moral Event Horizon crossover because there really does exist the possibility of things going wrong. Because of that, Moral Guardians and the like see it as evil, not because it *is* evil, but because they're afraid of the possible consequences. The Laser-Guided Karma killing off all the amorous couples in the series is a reflection of these fears.
Reconstructed: Alice and Bob become more informed about sex and how to protect themselves. They strengthen and stabilize their relationship. (And of course they strive to be good people.) Because they don't fear harm coming to themselves or their partner, sex is not seen as an evil act. It's good and beautiful. Or, at worst, as neutral as eating a PB&J sandwich.