> men cheat a little more than women, according to studies
Women admit to cheating less than men, and this is reflected in a bias of studies whose methodology includes self-reporting. In general, women self-report about one third less sex partners than men. That presents a curious mathematical problem: who are all those extra men partnering with?
Let's say the average reported number of sex partners for males is 11.7 [1]. Now that means that the set of men has a number of connections into the set of women times 11.7. Say, the number of men in the set is 31M [2], then the number of connections is 362.7M. For the set of women, the numbers are 7.7, 32.2M, and 247.94M, respectively.
The connections can terminate in either of the sets, so we can have M-M, M-F, or F-F connections.
At one unlikely extreme, we could have no heterosexual (M-F) pairings at all, so that means 11.7 gay partners for men, and 7.7 lesbian partners for women. Clearly, this isn't the reality, not even in the UK. But even if we map all the connections from the female set to males, we'll end up with 247.94M hetero connections, no lesbian connections, and 114.76 connections from the male set onto itself, or 8.0 hetero + 3.7 gay connections per male, and 7.7 hetero + 0 lesbian connections per female.
There may be a large number of spinsters, and a small number of gay people may be skewing the mean away from the mode for the males, but the important take-away is that, mathematically, hetero-sexual partnerships always draw from both sets equally, and therefore the mean number of heterosex partners is always the same across the whole population.
[1] http://www.nhs.uk/news/2013/11November/Pages/Results-of-UK-S...
[2] http://www.ons.gov.uk/ons/rel/census/2011-census/population-...