Happened to me. I wasn't so annoyed about the sleeping together part (it was over a year after we stopped dating) but I still felt betrayed.
What's worse is she told him all about my "quirks" in the bedroom.
Edit – bit more context. Dated the girl several months but in all honesty it wasn't serious. We ended things mutually but remained friendly afterwards.
The guy I had known for around 5 years and considered him a very good friend. He was also in a serious long term relationship he was unhappy with but too cowardly to end. She confessed to me one time she had been sleeping with him, which while I indeed don't own the girl, I still felt betrayed since the guy know full well I wouldn't go near other people's exes since it's off limits to me. Turns out that despite being in a relationship, the guy was "in love" with her and she ended it because "he had a girlfriend" (so she's ok to sleep with him as long as he doesn't get feelings, yeah).
I asked the guy and he denied it till I said she told me. He said he didn't want to tell me because he knew I wouldn't be ok with it. He then told me it was true and that she had told him what I was "into" in the bedroom. I then asked her about that and she vehemently denied it despite it being impossible for him to know what I was into unless she had told him.
I never spoke to her again. I forgave the guy but the friendship wasn't the same, we rarely if ever speak. He's still with his girlfriend but I haven't told her, I figured it would be up to him to come clean if that's what he really wanted to do.