My two sex with stranger types are:

One-night stands (or one-day stands).

Group sex with close friends, when sometimes the lights are turned down very low, or completely off. I've sucked a guy's cock while on all fours on the floor with my ass stuck up in the air, and sometimes a guy would slide his cock inside my pussy. Even though we are all friends, I never turn around to see who it is (plus it's very dark sometimes), so they would pop their load, pull out and walk away to go to someone else, while I'm still concentrating sucking on cock, not looking to see who fucked me.

With the parties, friends are constantly leaving and coming in, so the mystery guy who fucked me may not be one of the people I had seen earlier, when the lights were on. But like I said, I do know the person was a friend.