Women Cheat More...

... and Forgive Less

One in five said they would "definitely" have an affair if they fell for another bloke

In contrast, just nine per cent of fellas were certain they would betray their partner.

 and

But while 15 per cent of men would forgive a cheating wife or girlfriend, just 12 per cent of women would take back a partner who strayed.

It's seems to me, as long as people continue to do it two at a time (and I think that's still conventional, despite Hustler), that to achieve these kinds of results, you need more women who cheat with a small number of men, and a lesser number of men who cheat with more women.  Some sort of "conservation of sex" principle.