Isn't it part of the fantasy. You see the sexual things being done to the dick and it gets your horny because you imagine its your dick? Or rather fantasise about those things being done to you.
That's why most porn tend to minimize screen time for the men's faces. It's easy enough to imagine that that's your six-pack and fore-arm-sized dick, but the illusion is shattered when Rocco Siffredi's face shows up.
So I've always been confused by why it's odd that I don't like seeing nude dudes, but if you include a nude female or female doing sexual acts I'm excited about it.
Like people, it's not rocket science. She could be deep throating a goddamn banana and I'll get excited to watch it. Once naked ladies are in a scene I somehow don't even recognize that the actors are really in the scene. It's not that I suddenly like dick, it's that my attention literally can't see it.
As an example, in old school stuff they used to pan over to the dude plowing the girl, and only the dude was in the shot. That broke my attention and always had to skip it, cause, quite honestly, I ain't into being plowed by a dude.