I think their hypothesis is plausible, but I also suspect another factor: the increasing number of overweight and obese people who would feel uncomfortable showing their bodies.
So women are not showing up and the men who show up are old geezers...
So no young woman would want to be looked and stared at especially by the old geezers who are the majority.
It's the old "why we can't have nice things". Selfish Bad apples exploiting convention and ruining it for "good citizens"
It’s actually kind of the opposite: attractive women are less likely to be on nude beaches because they may get harassed by creepy men.
https://www.goodreads.com/book/show/4138.Naked?from_search=t...
Bobs Burgers - s03e11
If you (especially as a woman) are going to get pregnant, better make sure the father either is likely to have good genes, social leverage or will help you raise the kid, or ideally all 3.
Similarly (but less strong) for men, better make sure the woman is likely to get pregnant (she's not too old), is healthy enough to take care of any baby born (she doesn't have obvious physical defects).