I don't understand how this is exclusively a men's issue. Now this is completely out of my realm as a gay woman but wouldn't this roughly mean that there is a similar amount of straight women who are single as well? In my experience polyamory among the straights isn't nearly as common as the internet likes to make it sound. In general, millennials (and I assume gen-z as well) are having less sex on average and getting into less relationships than previous generations.
There's a Plenty of Fish or OK Cupid survey somewhere where heterosexual female respondents reported sex within the last year at a much higher rate than heterosexual men. The implication is that some of the more attractive men are having sex with more than one woman. There are some other surveys out there
In my personal experience, the average person having sex with more than one partner is unlikely to identify with the term polyamory. It's not as mainstream as the internet makes it seem.
I'm not making an assertion about that. Could be either, imo more likely the latter. There are people I've met who are basically poly but balk at the label.
There’s studies that show heterosexual men are happier in relationships; while heterosexual women in relationships are the least happy of all the relationship demographics. So I think there’s more single het men because there’s just more het men per capita. But even if there’s a similar number of single het women it has drastically different implications on quality of life.