Straight acting homosexual male.
Wow! He's such a heterofag!
A heterosexual fag. "Fag" originally was/is an offensive and hateful term for a (mostly male) homosexual, the word has evolved much like the terms "moron" or "idiot" which originally meant a medically handicapped person, but is now a widely used term for a stupid person (and not considered offensive to mentally handicapped persons. The word retarded is now going through that same change, and will within a generation most likely not be considered offensive to the mentally challenged). The actual word "heterofag" is not used, but is simply shortened to fag. Hard to precisely define, but kinda like a tool.
"Do you know Bob?"
"Yeah, he's a fag."
"No, he's got a girlfriend. He's just a fag."
"What's a heterofag?"
"A person who's a fag, but who may or may not be homosexual."
"Well what do you mean by fag then? If you're not talking hatefully about a gay person?"
"Closest thing would be a tool. You know a fag when they're being a fag".