Have one time been at a party with a gf and girl I'd previously slept with. I didn't reach out, and past girl took the hint that I was with a gf. No harm, no foul.
That assumes you don't sleep with crazy girls though that might come up and intentionally hint at things to get at new girl's ire. <<< It's that kind of crap that is the reason I would never hang out with guys that have slept with my gf. The more I think about it, the more ludicrous it sounds that this should be accepted. Girl would have to be really stubborn to bring you around that, or just completely oblivious.
Dealt with that once. Big group "oh, no that was a long time ago, I'm not even remotely interested, and he knows that". Drinking commences, old guy starts trying to dance with her and even is looking over her shoulder at me to see my reaction. How about not only am I not going to step into your testosterone challenge, I'm going to say fuck it to the girl who clearly doesn't realize that these guys still look at her and are trying to challenge me to "compete" for her. Chick had almost all guy friends, and 3 of them were exes. What a dumb bitch that chick was lol. Sharing a girl's time with her exes is not my idea of a relationship. So glad I took the high road and stopped talking to her.
Did not reach sleeping with status, probably saved my junk.