More Sex Is Safer Sex
If prudes occasionally slept with strangers, it would slow the spread of STDs. Here's how it works. One such prude walks into a bar, and he's uninfected. If he takes home an uninfected woman, great - he distracted her from a potential disease carrier. If he gets herpes, that's also great, because he's sexually conservative and won't pass the infection along very often. Better him than someone with less self control.
Either way, society benefits when the chaste open up slightly. "Slightly" is key, because too much "openness" spreads more disease than it diverts. After studying AIDS in England, Harvard's Michael Kremer put the cutoff at 2.25 partners per year.
Meanwhile, Harvard remains the fleshpot it was when I went there.