I think that a lot of gay men are stupidly fooled by the tag "straight" on a lot of gay porn. Websites like Sean Cody, English Lads, Seduced Straight Guys all do this. However, one model of SC said he was asked to claim he was straight or bisexual (I can't remember his name. You'd have to search it) . On English Lads, a lot of models (like, for example, Jaden and Ben) are said to be straight but are actually bisexual escorts in real life. I very much doubt that the majority of men said to be straight in these kinds of videos are actually straight but bisexual, in the way that actors like Marcus Mojo and Jimmy Clay are, or just gay (God forbid gay porn actors just be gay! How dare they.)
Some websites seem to actually be real, like Seduced Straight Guys and websites where straight men are "serviced", for example. It's an exciting fantasy that has become over-saturated recently due to the subculture becoming more assimilated with heteronormativity, I think.
I would have sex with a woman if I were paid to. A lot of men don't really care who they fuck as long as they get enjoyment out of it. I don't think sexuality is really just about sex but other things like desire and emotion. There are plenty of men in porn who might fuck for cash someone of a gender that they wouldn't sleep with or date outside of the industry.