To answer your questions from the duplicate thread - I'd be upset if my partner was doing something sexual and deliberately not telling me about it. The idea of them realising I might not like that they were doing a thing, but not telling me about it and continuing to do that thing, would be really undermining.
I don't think your boyfriend is necessarily not-straight, though. People seem watch all kinds of porn that don't necessarily match their orientation. I see plenty of threads on here about straight people watching different sorts of gay porn and being really confused and scared by it, and wondering if they're mistaken about their sexual orientation - and some of them seem to turn out bisexual or pan, but plenty of them do just seem to be straight but enjoy gay porn. At the end of the day, if he says he's attracted to you, you'll have to take that at face value.