Sexual Weirdtopia: It's illegal to be a virgin past a certain age, say 25. Each person must show proof that they've had sex at least once before their 25th birthday, or face punishment (which could range from a fine to execution, depending on the level of dystopian-ness desired). Stories could deal with the difficulties faced by unpopular or unattractive people in meeting this deadline, or with the complications entailed by the requirement of proof.

An interesting variation would be for the rule to apply only to one sex, say males.