The only hentai game I played through to the end was Hunie Pop.
Nearing the end I got an alien artifact, and showed it to Nikki, the geek girl who was a UFO and alien-life enthusiast.
Look, Nikki, actual material evidence of extraterrestrial intelligent life. YOU WERE RIGHT ALL ALONG! Sadly, Nikki gave me the canned ERR: Not on my designated list of acceptable gifts response. Disappointed.