If guys will fuck sheep, they'll fuck anything. My experience with married guys, though, is that they want to get fucked up the ass. They can get all the pussy they want at home.
Human sexuality is far more complex than simply who one has sex with. It's a combination of multiple factors. For the same reason that a gay person can have sex with someone of another sex and still be gay, so too can a str8 guy have sex with a gay guy and still be str8.