Personally, I’d say it goes just a bit deeper than that: It’s about what you find attractive about it. Some straight people watch transgender men have sex with cisgendered women because it’s just a little more woman than straight porn. At the same time, some lesbians will go through mental gymnastics to justify watching some man-on-woman porn because of a fear of not fitting into the sexuality they want. The idea is to understand your sexuality through what you like and don’t like about each variety; The porn you watch doesn’t choose your sexuality for you.