Conservative
Type 40
"Dude! You had a threesome with my girlfriend and my little sister! WTF?!"
"Is it illegal?"
"Why, no it isn't. My bad. Sorry, for getting out of line there."
Since YOU brought them into it, the rules state it's open season.
If they don't have a ring on, they are both fair game. Not someone else's problem if you can't satisfy your GF and she goes looking elsewhere.