the fact that you would consider cheating to fulfill your fantasies make me not like you very much, but my advice, masturbate all you want. Fantasize all you want, cuz gay porn and real life gay sex isn't the same thing. Sex between two people isn't a game, unless your some kind of slut. Cherish what you have and stop being selfish. If you wind up leaving this girl and you say hey i'm going to look for a guy, be sure you can be with a guy. If you want a hook up and get a nice STD, try Craigs list

I would say it depends on how turned on you are by the thought of doing something with another man. If it turns you on quite a bit, I would try havign sex with a man. I would deffinately try it before things get more serious then they allready are with your girlfriend, that will just cause more issues. Good luck man!