Well, you don't need to know any openly gay men. Just write a normal guy. Maybe a bit more conscious of appearance than a typical dude - my buddy Jaime reads GQ and uses moisturizers *before* he actually gets chapped lips and hands, for instance, compared to my standard-issue nerdy slob approach - but there's messy, grungy gay dudes too. The only real difference, on average, between a random dude and a random gay dude is A) Who makes them turn their heads and go, "Damn!" and B) How willing/able they are to do that in public without worrying about getting punched. They're just dudes from any and every walk of life and they run more or less the same gamut of personalities. Some guys are masculine, some are really hyper-masculine, others are more what you'd see as a "stereotypical" gay man, etc. Everyone's got a mix of traits in them, though; Lord knows I've been mistaken as homosexual more than once, yet I have almost no interest in actual same-sex relations. In the same way, a gay man might be really, really exaggeratedly "gay" or he might be nearly undetectable until he tells you himself.
If you really feel like you can't possibly write one without any real-world experience, look up your local gay hangout and spend an evening drinking and observing quietly. You can watch any number of casual interactions in an atmosphere of minimal fear. Just don't, like, take a notebook or start loudly announcing that you're not gay and are just looking for some information.