I had that problem pretty hardcore when my boyfriend started dating his girlfriend (now our girlfriend) 4 years ago. It was really visual and very upsetting for me. I felt like it was "gross" even if he showered afterwards. I don't know what to tell you other then I made some rules about it (no sex with both of us on the same day, for awhile I wanted at least 48 hours between him having sexual contact with her before we could have sexual contact) and then it stopped mattering. I don't really know why. As I got more comfortable with them as a couple and stopped feeling like they stopped caring about me when they were having sex and got to know her more and felt safer expressing my fears and worries to both of them... the feeling faded and then was gone completely.
For me it was just a very specific manifestation of the things I wasn't really okay with yet. So I made some rules but I watched them closely and kept checking in with myself if I still needed them and one day I didn't.
Rules and limits are a kind of crutch for me. I use them to help when I'm not strong enough yet. They are extremely useful as long as you are willing to keep checking in and not rely on them to much.