Roommates with benefits.
Me and my roommate at uni were pretty notorious for striping at a party one time and going down on each other. What the rest of our uni friends don't know is after that it became a pretty regular thing. It was the reason me and her ended up sharing a room in second and third year even though we didn't have too.
I haven't been with another woman since that time but I'd like to meet up with her again for more than just old times sake.