A Freakonomics-like approach to sex and promiscuity:
Imagine a country where almost all women are monogamous, while all men demand two female partners per year. Under those circumstances, a few prostitutes end up servicing all the men. Before long, the prostitutes are infected; they pass the disease on to the men; the men bring it home to their monogamous wives. But if each of those monogamous wives were willing to take on one extramarital partner, the market for prostitution would die out, and the virus, unable to spread fast enough to maintain itself, might well die out along with it.
