Is Hentai Illegal in the United States?

Not that i look at that stuff...no,never (no sarcasm), but i was considering doing the subject for a project at school. Essay stuff, y'know. But no site that i have come across yet has given me a definitive answer with evidence to back it up. I'm not asking about lolicon, or any of that other gross stuff that is so obviously wrong and illegal. I'm asking about pure, man and woman, hentai. Some good law sites would really help.