This is an interesting but complicated topic. I will not be giving it the full analysis it deserves.

Oversimplifying and freely admitting that I never did it but have occasionally mildly fantasized about it, I see Forced Bi having two basic forms

The first is that it is a mostly maso X forces one to do Y fantasy where X involves a lot of pain while Y is something that makes you psychologically uncomfortable

In the second form the sub is Bi Curious but camouflages that with an elaborate role play which is less masochistic and more submissive than the first version

If some sub has done it and thinks he fits in neither of those 2 categories I would be happy to hear his phenomonology

Turning to gays I think straight sex lacks a repulsiveness similar to what gay sex has for many straight men. Quite a few have actually experimented with it. It's just that they derive little or no enjoyment from it. In this case Y is oh so boring. For the minority who do find it repulsive it's usually due to gay misogyny rather than a cultural taboo.

That's from the perspective of a participant. From the perspective of the porn viewer if he is the type who thinks straight sex is blah he's going find the clip uninteresting. If he is a gay misogynist he is going to be angry.

Interestingly a few years ago I read a good sized essay who divided gays into groups. The criterion he used was to have them watch a clip which was purely gay sex but in which a woman pops up in a nonsexual way. For a majority of the viewers the woman was irrelevant to their enjoyment. But there was a sizable minority who felt she completely ruined the clip for them.