The thing is that a majority of viewers who watch hentai are men, and (straight) men prefer to look at girls. While there are some homosexual men who watch hentai, the majority of its viewership are straight men. That being, the straight men who watch this stuff don’t really care much for the guys banging the girls. They’re literally just there because a floating penis raping a girl steps into the realm of the supernatural, a whole other genre of hentai. That’s all the purpose they serve, and so animators don’t have to work as hard to draw them. They focus all their time on the faces and bodies of the girls being raped because that’s where the spotlight is.