Straight men don't go for that. It's either catering to closeted gay men or bi guys. I've heard that out gay men generally don't go for tranny porn too.
So either it's customer base is closeted gays and bisexual men, or the directors have a disconnect between what they want to see and what the supposed fanbase wants.
I dunno. I would have thought that anal sex is kinda gay too. Yet lots of straight guys here are obsessed with it.