every time a male has sex with a female, both of their opposite-sex partners rise by one.

Just to ensure clarity, you meant to say; "every time a male has sex with a new female [partner], their opposite-sex partners rise by one. Correct?

One other thing which could skew the statistics is the fact that people that have had many sexual relationships can die, and the dead are not often counted in statistical surveys, while some of their partners might be.