See e.g. some U.K. data here, or the U.S. data here (which conveniently control for race, so that the trends are strikingly obvious as a class phenomenon).

Thanks! I think I misinterpreted your earlier post; when I wrote the grandparent comment, I had read 'monogamy' as you referring to faithful long-term one-on-one relationships, not just the subset of those relationships that are marriages. But it sounds like you mean marriage proper, in which case I think you're right (albeit depending on what scale of 'environment' we're talking about).

However, even regardless of any research data, things should be obvious from common knowledge and everyday observations. There are clearly lots of men around for whom getting into any relationship with a woman would be a Herculean accomplishment, even more of those who struggle with positive but still meager results, and a minority for whom getting laid with attractive women is almost trivial, who easily rack up many dozens, hundreds, or even thousands of notches.

If I'm honest, my own everyday observations and knowledge don't seem to be strong evidence of all three of: (1) breakdown of monogamous norms, (2) a concurrent increase in female hypergamy 'where a minority of exceptionally attractive men account for the overwhelming part of non-monogamous sexual pairings that take place', and (3) a causal relationship between #1 and #2. It doesn't help that I didn't even hit adolescence until last decade - I expect I have much less experience of relationship trends over time than you.

It seems plausible to me that men have a much wider variance in heterosexual sex partners than women, but I'm not sure it's definitively confirmed - or that the variance ratio has increased over the last few decades because of a decline in monogamous norms. The PNAS article suggests that a failure to account for prostitutes in sex surveys can explain the male-female difference in mean number of sexual partners, which hints that it might also explain the male-female difference in variance, too. (After all, excluding female prostitutes from a survey is a little like chopping the right tail off the female sex partner distribution, which would decrease its variance.) The other review article doesn't seem to suggest a clear trend in the male-female variance ratio over time, either, although it looks like there is (surprisingly - at least to me) little high-quality data for judging that.