You're presenting a false dichotomy, heterosexual men never had men as "eligible" partners and them could simply not engage in long term partnership as opposed to change sexuality, but you've always been a poor poster who plays obtuse when called out on half cooked arguments
Your argument suggests we're frozen in time, it suggests the way people have identified and paired yesterday will be a constant and a predictor of how people will identify and pair tomorrow.
There's a growing population of men who call themselves heterosexual but engage in sexual encounters with other straight men, an idea that isn't in line with contemporary societal norms but also isn't unheard of as we can look as far back as Ancient Greece to find similar examples.
If modern society continues to implicitly push the idea that only virgin women are worthy of companionship while simultaneously encouraging the sexual liberation of women in a context where a different standard applies to men, a foundation is being laid that leads to men only using women for sex and concluding that's it's not an indictment on their masculinity it sexuality to seek companionship with the only population that doesn't get a bad reputation for having sex with multiple partners, other men.