I've been on a mailing list for wives of gay husbands (the wife not knowing that her husband was gay when she married him). The stories are generally pretty similar... the guy liked the girl, thought that maybe he could be straight for her, etc, maybe even didn't mind sex (hey, if your own hand can feel good, so can a pussy)... but pretty soon, he became more distant, they had sex less often, the wife starting doubting her own attractiveness, her own ability to be a good wife... then some day she finds out he's gay, either because she catches him cheating, or because of tons of gay porn on the computer, or because he breaks down and tells her. This happens after years of being unhappily married... for some after a couple of years, for others after four decades. Lots of depression and anxiety and all that that could have been avoided if only the guys had realized that they could not be straight, no matter how hard they tried.
So, if he's gay, and not bi, then please do yourself a favor and find some straight guy to date, because even if you can lure him into dating you, it might be fine for a little while but it won't work out in the end, and everyone is going to be worse off.
I do agree that people aren't usually 100% straight or gay, so to some degree everyone is bi, but as someone on the wives of gay men list said, her ex husband enjoys sex with women, but doesn't miss it when he doesn't have it, whereas when he doesn't have sex with men, he craves for it. So if he's bi but mostly into men, it's better to just let him find a man. There are plenty of other men out there for you to date who are mostly attracted to women.