I think it's more of a psycological thing. That's why almost ALL guys think about it. You're a guy, you're "pleasing" you're body, feeling it and stuff........., you like it so you imagine others, not nesscarily them individually (their faces/ personality), just their bodies. you're probably attracted to the male BODY which is a little different than being gay. You just have a really strong sexual appetite.
if deep down when you think about it, you're not sexually aroused ( and it makes you sick to think about being with a guy), you are ABSOLUTELY NOT GAY. I know cause I have the same feeling about girls ( I'm gay). It's not really a "sick" feeling, but I don't feel any SEXUAL attraction.
Really belive me, you have NO WORRIES. You aren't gay.