The first "adult game" I owned was Sex Games for the C64. I believe it was in French or something.. instead of Yes it said JA!. You had to jiggle the joystick to make people have sex.
When I was in 7th or 8th grade in high school we'd sometimes stay after school to use the computer room. One day behind the trash we found an Apple II floppy. On it was old animated porn. Girl buys loaf of french bread, then sticks it up her you know what. Black guy with straw hat sitting next to tree getting sucked by a girl and his toes tingled, etc. The floppy was damaged, though, so every time we used it, it would work less and less. We couldn't get it to copy either. Finally it stopped working totally.
Anyone ever played Peter Packrat? Great arcade game! I never saw a real machine, only in MAME. There's a few other really good games I've found in MAME, but never in arcades. I can't remember the names, though. One I enjoy is Jumping Jack by Universal.