I'm going to get a little philosophical here...
I don't think it's gay if you are a male who gets excited by shemale porn. In fact, I don't think it's gay to enjoy any sexual act, no matter how 'gay' society may label it. For instance, does it make a man gay to enjoy the physical feeling of having a vibrator inside of his ass while he masturbates? No, it doesn't. And, if you think it does, allow me to prove you wrong - what if, instead of masturbating, that very same man is having sex with his wife and his wife is the one happily using the vibrator on him? How is it gay for a straight man to have sex with his wife and use sex toys? The answer - it's not. It's not gay. Because, gay is a type of sexual orientation - a male who is attracted to another male, a female who is attracted to another female. It is not, however, a way to accurately describe a type of sexual gratification.
Think of it this way - if it is gay for a man to enjoy shemale porn then it is gay for a man to enjoy watching porn that involves a woman receiving a facial or even a man having sex with a woman. I mean, if you're a man and you get off to watching an erect penis squirt cum or if you get hard watching another dude's dick fuck some random chick then you have to be gay, right? Wrong. It's not gay - it's just a turn on. It ends there. No more, no less. If it made a man gay to watch that stuff, then 99.999999999% of the people on this forum would be flaming homosexuals.