Let me explain.
In porn, the “lesbian” category is inside the “straight” section and women are expected to play with each other in threesomes. But if men play with each other it is “bisexual” and locked inside the “gay” section. It is a weird double standard nobody thinks a about.
that’s tough to answer because desire is so personal. nobody understands the reasons they like watching certain porn, nevermind other people’s reasons.
there must be something primal there, maybe linked to why polygamy is much more common than polyandry. in sure you can find serious essays and studies about it.