The Economics of Sexuality:

A man who is sent to prison finds that the price of sex with a woman has spiked – talk about a supply shortage – and he becomes much more likely to start having sex with men. The reported prevalence of oral sex among affluent American teenagers would also seem to illustrate price theory: because of the possibility of disease or pregnancy, intercourse is expensive – and it has come to be seen by some teenagers as an unwanted and costly pledge of commitment. In this light, oral sex may be viewed as a cheaper alternative.

(Though I admit skepticism of anything associated with the Freakonomics guys)