The game is pretty sexual in nature. You're basically cheating on your girlfriend Katherine with a girl named Catharine. Then, while you sleep you have nightmares which lead into the gameplay - it's a puzzle game. You have to climb to the top of the mountain, pushing blocks and figuring out patterns. Sometimes a giant baby tries to kill you. Sometimes it's boobs. Or a giant ass monster with a tongue coming out of the crack...yes, I'm serious.
I don't remember any nudity or hardcore sex, but the whole game's story is built around sex. So while not hentai-ish to the point of pointless sex, the ideas of commitment, sex, lust and its consequences ARE the point of the game.