My gf had a friend she had hooked up with multiple times. There were times where the friend made out with me (me as in the bf) and my gf and then the girls would go at it and let me watch and then get involved and have sex with them. Both would let me kiss their breasts and touch them, but then during the middle of one hook up session the girl stopped being interested.
She stopped kissing me and my gf would kiss her but the girl wouldn't really kiss back. Everyone was completely naked and the girl wouldn't let us finger her or anything! It was just weird because of the other times she was really cool with it, but this time she just stopped.
It was sad cause she was pretty hot but I think she was just wanted to try it out and that was the final experience for her (or someone probably said something and she did not approve?)