Futanari is the least gay porn

If you're bi (or just curious), it's a way to see dicks in porn while still getting a pleasing female figure with breasts and a pretty, feminine face and whatnot. If you're into femdom, it can be appealing because taking dick is often seen as a submissive thing (and in real femdom, you often see the dominant woman using a strap-on for this reason).

Also, to put a less serious spin on things, it's arguably the least gay 2-person porn possible. If you watch straight porn you have 1 guy and 1 girl, meaning it's 50% gay no matter your orientation. Futa involves 1 girl or guy, and 1 half girl half guy, meaning it can be 25% or 75% gay, depending on the flex slot. Having 2 guys or 2 girls makes it 100% gay no matter how you look at it, because it's literally gay sex.