I know that in the mid-late 90's, there were a slew of interactive movie-style porn games (housemate in college collected the damned things, the little spankmonkey).
Don't remember if they were AO'd. I do remember that he ordered them all off the internet, so they might have avoided the rating by being a direct product.
For retail, the only game that I recall as being worthy of AO was Phantasmagoria II. Kind of surprising considering Sierra and Roberta William's past projects.
And of course there were the MegaTech games (eg. Dragon Knight), but that was more silly anime nudity than anything.