Why aren't there more bisexual men?

This is a discussion post as a prelude to a more meaty thesis I've been developing and will post here in the next few days.

There were many historical societies, like Ancient Greece or feudal Japan, which had societally accepted (expected, even) bisexuality between men. For instance, the Greek city state of Thebes was famous for its elite fighting force called the Sacred Band, which consisted of 150 pairs of adult male lovers appointed based on merit - they were not screened for their sexual preference, it was just automatically assumed that if you were an adult man, you were down for getting it on with other dudes. The Sacred Band was famous because it was said that having their lover next to them on the battlefield made them fight much harder than any other force.

Homosexual behaviors among men were so accepted and talk of it so commonplace during that period that Plato wrote a dialogue called the Lysis where Socrates visits a wrestling school for young men and counsels one who is head over heels for a fellow student on the socially proper way for a man to court another man, specifying that feelings of eros - erotic love - arise naturally between two men who are close.

These people weren't a different species or something. They were the same kind of people as you or me - which seems to suggest that, absent societal conditioning, men tend to be a lot more bisexual than we'd otherwise think. If that's true, then why, in our age of supposed sexual liberation, do we not see more men exploring sexually? 21% of Gen Z women identify as bisexual - but only one third as many men - 7% - do. Bisexual identification of women increased by 12% between the millenial generation and gen Z, but only by 4% for men.

I think this question has important implications for men's liberation and the ways in which heteronormativity shapes and suppresses men from developing their sexuality freely.