Pasted from another thread.
'I am kinda at the other end of this than you. I am a single woman who is developing a FWB type relationship with someone who is married. All above board, everybody knows, blah blah.
But I have been feeling very uncomfortable and weird about seeing him, about being touchy with him, even though that is totally fine with both of them. She seems to think I'm a decent person. We're not interested in each other sexually but I like her and enjoy her company. He and I are going slowly, talking a lot and so on. They are not the issue.
I realized that I feel guilty. I have never been involved in any way with someone in a couple. And it feels so strange to me to touch someone else's husband, especially when she is around and might see! And I still feel weird when she isn't around - just not as much. Clearly some monogamy programming to deal with. So I've decided to talk to her more directly. We've talked but not explicitly about the developing FWB with her husband. I think that will help with the guilt. And I continue to feel it, acknowledge the weird, and try to move on.'