Here is a very interesting chapter from an upcoming book by Steve Landsburg, in which he argues that sexual conservatives will actually reduce the spread of AIDS by having more one-night stands:
I don't think this blog is the ideal place to debate the matter, but my own feeling is that Landsburg is basically wrong. At least two major problems:
(1) He assumes that everyone is purely selfish, e.g. Landsburg's model (it seems to me) would not include the possibility of someone discovering he is HIV+ and then refraining from future sex because he doesn't want to hurt other people. Once we realize that there are such people, then I don't see why Landsburg denies (at the end of his chapter) that his analysis might encourage more promiscuity. ("I'll be doing the world a favor by going out tonight! A Ph.D. at the U of R even says so!")
(2) He assumes that the degree of promiscuity is a purely individual decision, rather than one formed by culture. To take his "monogamous wives" story, once the women all take on an additional partner, then (I claim) the next generation of people will take this behavior as standard, and then future Landsburg's will argue for even more promiscuity still.
I realize I sound like a stodgy Puritan who rejects economic analysis, but I really do think the analysis is dead wrong in this case.