I vaguely recall an old sci-fi novel with a similar setup, and no it wasn't hentai. It's a really old story, maybe short story. A guy from the 40s or something like that takes a time machine into the far future, and humanity has solved most of our societal problems. Also humans have evolved into hermaphrodites. There is supposedly no more biological sexual dimorphism - everyone has a functional penis and vagina - except instead of obvious genitals on the outside, the penis has turned inward into the vagina, where it can move at will and allow a person to inseminate themselves. (actually don't remember if they all had uteruses or if they were infertile and used technology to reproduce) I believe the twist was he finds out humans hadn't evolved into that. We had turned ourselves into hermaphrodites with technology, like the Mochlans in The Orville, and also like the Mochlans we had become disgusting sexual degenerates. In the future everyone is basically masturbating in public all the time. The guy from the past ends up absolutely disgusted and disappointed with humanity and wants to return home as soon as possible.

I probably got a lot of the details wrong. Just remembered thinking it was ahead of its time.