Well isn't there less sexual fluidity in men overall? This is not my area of study (I'm a linguist, not a sex researcher), but I have dim memories of reading something to that effect in an academic journal article (probably dealing with the sociolinguistic properties of 'gay speech' or 'gay accent', which is something we did a week on when I was in grad school).
That might explain what you've called the 'threat' of not being 100% heterosexual. Because most men
are 100% heterosexual (or at least 100% heterosexual sexuality is statistically more common among men than women), perhaps they become worried because there is then something that separates them from the norm? Nobody likes being out on a limb, after all, and if they're not 'fully' gay then it is understandable that they might feel like they're in a bit of a (apologies for the unintentional pun) no man's land.
At any rate, yes, it is more complicated than it seems. My only point was to say that for mature, adult males the problem is not in recognizing that other men are good looking, but in
comparison that makes us feel bad. We could be having the same conversation about salary, or housing, or marital status, or anything else, and some guy who has none of that would feel bad, and some subset of those guys would take it out on the world in 'homophobic' or otherwise concealing ways.
We don't want to admit that when we've failed, basically. You probably know all the old hackneyed jokes: men don't want to ask for directions, men don't want to read the directions to anything, men don't want to XYZ, etc. All of that is based in at least a grain of truth, and it extends to our interpersonal relationships with other men, too. We don't want to admit that someone has something (genetically?) that we don't. "Eh, he's okay...I mean, Leonardo DiCaprio...come on...he's
kinda rat-faced, isn't he? And George Clooney is a boring old man." Okay, then. Thank you, ego. You can sit down now.