You raise a lot of important issues, here. To offer up some of my own thoughts, for what they're worth (not being a straight guy, myself) --

1.) I think a lot of straight men who are our genuine allies seem to be insecure because of their own lack of girlfriend/wife, so they want the entire world to know they're straight so heterosexual women will be aware of their potential availability (rather than assuming they're unattainable to begin with).

2.) Some straight men may have had negative experiences with gay or bisexual men who sexually harassed them or couldn't take no for an answer. Of course, the onus there is on those gay/bi aggressors to cease their predatory behavior...but the straight men who were victimized by it shouldn't proceed to take it out on gay or bisexual men who are cordial/passive/nonthreatening.

3.) For straight men who are already married (or partnered with a woman), it's homophobia (or heterosexist orientationism) stemming from their hateful/indoctrinated hearts. And there's a version of this that applies to single heterosexual men, as well. (e.g. many "incels"?)

4.) I've also encountered straight men who are cool with me and comfortable around me in a brotherly way (some even being comfortable changing clothes or getting completely naked in front of gay/bi dudes, sometimes to playfully tease us)....but a lot of that sentiment can be attributed to how they don't view me as a "threat" to them, in terms of being potential competition for the romantic affection of heterosexual women.

I'd imagine it's different for every heterosexual man...and that's, of course, not even including the ones who have very healthy attitudes toward LGBT peers and share camaraderie with Queer men while being straight allies...and these are obviously the ones who don't feel the need to insist how heterosexual they are, at every turn.