Are you saying that you think your relationship is a sham, or just that you are concerned he might cheat? If the latter, has he promised to be monogamous? A bisexual man is just as capable of sticking with a promise to be faithful as a straight man is. If he has promised fidelity to you and you don't have a reason to think he is cheating, you should probably believe that he can keep his promise. (I will add, I have never been one to think that looking at porn is cheating.)
Can you be satisfied with that, or is it a game changer for you, even if he were to remain faithful to you?
There IS a difference between "gay" "homosexual" and "bisexual." The three are (and, this is in real life) NOT the same. Maybe he's bisexual. Many (if not most) bisexual men get married, they look and act like any other man, and their cavorts with men are never revealed. They don't necessarily stay married forever but neither do straight guys these days. Most never ever approach the wife (or girl friend) for a three way with another man, but a few do. But now you know. BTW, the term "gay" is not a sexual orientation. It is a socio-political orientation (and a made up euphemistic word). The word you're asking is (he) "homosexual?" If he's hitting the sack with you he isn't homosexual. He's, if anything, bisexual. A homosexual man (that's the 3%) won't willingly get naked with a woman. And there is an infinite spectrum between the two.