"I'm not bisexual, I just like sex with both men and women."
It's just a theory I have, but I'm guessing that men who have sex with other men, but claim to be "straight," are insecure, and think that being gay or bisexual makes them "less of a man." Yet another discreet form of homophobia.
As long as you think of gay/bi as "other," then you can rationalize that you are neither because you are not
different no matter what you do. That seems silly, but even sex researchers have trouble figuring out how to classify men in prison, boys in private schools ("public" for Brits), Military dudes, etc. They are having sex with each other, but allegedly only because there are no women available. I'm not defending it, but just noting that even at very academic levels the whole idea of non-conforming sex practices get tricky.
In fact, the whole idea of "sex" is tricky to classify. If a man and a women masturbate each other are they having sex? (Pretty clearly, yes.) If they masturbate themselves for the titillation of the other while occupying the same bed are they still having sex with each other? (Harder to say.) What about if they masturbate themselves while in the same room, but not in the bed still of the titillation of the other? (Harder yet.) What about if they masturbate themselves while on opposite sides of a glass particition in a peep show, still for the titillation of the other? (Hardly anyone would agree that they are still having sex
with each other.)
The point is that neither orientation nor even the basic idea of "having sex" has hard and fast demarcation points. I make fun because personally I think most of these fussy notions have to do with our cultural discomfort with sex in general, but one is not really crazy if one can't decode our incredibly ambiguous definitions.