Hi!
I think that there are several factors in this.
One strong factor is creating a minority group. As with any discrimination, denouncing another group is a fast and easy way to raise yourself in your own perception. You know, an attitude like :You have no job, you drink more than you should, your wife is ugly, and you beat her up regularly. But at least you are not a [censored] queer!! In other words, as long as you can find someone whom you deem even less prestigious, you can't be the bottom of the hierarchy. So, such homophobia can be an expression of low self-esteem and strong doubts.
In a similar vein, I have the suspicion that there is a strong sexual insecurity among many heterosexual males. I mean, all this boasting and so on - if you are confident in your manhood, you don't need to present it to others and get their confirmation. I am a straight male, I know that, and that is the end of that. I don't need to pretend to be "a great stud" or "as hot as hell", since I know what I am. Especially when you see how fearful they respond to being brought into a context that may make them look feminine, such homophobes show their colors: I mean, if you really are a man, you will still be a man, even if you wear an apron in the kitchen. Somehow, I have the impression that those men actually fear they may find out that they are actually homosexuals themselves and thus avoid anything that could drag them "to the other side" (like a former alcoholic should avoid alcohol).
And not wanting to flirt or have sex with a male is normal for a heterosexual male. That is not homophobic, but part of the definition (if you enjoyed flirting/having sex with a male while being a male, you are either bisexual or homosexual). Homophobia starts where you speak out against things that do not influence you directly (flirting and having sex usually do influence the participants).
Deathworks