There's a perception floating around that men tend to have more sexual partners than women, because, you know, men are horn dogs. You probably believe it yourself. The media certainly does.
By virtue of the fact that sex has to occur between two people, it's mathematically impossible for men to average more sexual partners than women. For every promiscuous man out there, there has to be a promiscuous woman willing to sleep with him. So, when you take a national average of heterosexual sexual partners, as all of these studies have done, the number of partners for men has to exactly equal the number of partners for women.