From The New York Times:
Take the question of promiscuity. Everyone has always assumed — and early research had shown — that women desired fewer sexual partners over a lifetime than men. But in 2003, two behavioral psychologists, Michele G. Alexander and Terri D. Fisher, published the results of a study that used a “bogus pipeline” — a fake lie detector. When asked about actual sexual partners, rather than just theoretical desires, the participants who were not attached to the fake lie detector displayed typical gender differences. Men reported having had more sexual partners than women. But when participants believed that lies about their sexual history would be revealed by the fake lie detector, gender differences in reported sexual partners vanished. In fact, women reported slightly more sexual partners (a mean of 4.4) than did men (a mean of 4.0).
So how sketchy is the research on human sexual behavior, anyway?
If you're counting unique partnerships rather than acts, around 30-50%. Study here; analysis of some of the flaws by a retired whore here. There is, unfortunately, a lot of educated-guesswork involved, criminalization being what it is.
NSFW, obviously. Well, those specific links are probably okay, but the site as a whole isn't unless you have a very understanding boss.
Considering that most "unique partnerships" with prostitutes are likely to be, well, shorter than average ...