I'm sure these articles have been linked here before, but they're relevant to your concerns:

https://www.theatlantic.com/magazine/archive/2017/09/has-the...

https://www.theatlantic.com/magazine/archive/2018/12/the-sex...

>You list men as the reason why people have less sex, but unless women is having a lot of more sex with other women to compensate the numbers,

Fewer men are sleeping with more women.

The source for the article cites both women and men. That said, if it is also true that fewer men are sleeping with more women then the cause of this can also be attributed to the choice of the women than the choice of men.

another possible explanation is that fewer men are competing in the dating pool at all

but assuming your hypothesis is correct, to what do you think that choice can be attributed?

If we look at Japan as an example, the observation I would make is how much people interacts with computer that simulates or replaces physical social interactions with a computer facilitated one. Be that a social networks, traditional media, movies, tvs, but also practical every day activities like work, shopping and entertainment. This interferes on a psychological level how people form emotional bonds, which changes behavior in both men and women.

If we observe that women are choosing from a smaller pool of men, one way to describe that would be that women turning into more tournament-like in their strategy for reproduction and away from pair bonding. Tournament and pair bonding strategies have both benefits and drawbacks, and both are related and possible influenced by perceived social status of potential mates.

that sounds like the cause being fewer men being in the dating pool, like I was guessing, compared to what you said before:

>the cause of this can also be attributed to the choice of the women

TIL that social media is acting as birth control as a second order effect.