ASSERTION: Shemale-on-female porn is straighter than male-on-female porn; you're essentially watching a woman with a life-like dildo have a go at one of her lesbian friends. Both "actors" have the female properties one typically looks for in such films, except that there happens to be a cock on one of them (you'll be watching cock in the male-on-female film as well, so this is immaterial).
All things being equal, isn't this less gay than watching another man get laid, complete with constant shots of his salty ballbag, hairy ass, his face upon completion, and so forth?