The main issue with the whole 'we're living in an increasingly harem culture' narrative is that it just isn't true.

GSS data from different birth cohorts, number of sex partners

What we see here is that if you were a young adult (20-40) between 1960-1990, you were getting laid more.

The share of men with 50+ partners has gone from a low of 1% to a high of 5% to roughly today's 2%.

But hey, you could say that 'dude, it's not the combination of unique factors in the baby boomer generation that did it, there's this new trend that is causing this new shrinking of the top 5% or so...'

Then we can look at 'how many partners in the last year' variable:

http://imgur.com/a/jQodnWh

Look at that. The share of dudes getting 5+ partners a year has remained pretty stable. It's the top 5%.

If you think the GSS is somehow inaccurate, you should question that assumption:

http://www.nicholaswolfinger.com/blog/2019/04/02/evaluating-the-accuracy-of-general-social-survey-sex-data/