When I first met now-hubby (again, I want a nickname for him, but nothing really sits well, and I've already used that term, so thus shall he be), a large obstacle was the open relationship. I had made the conscious decision when the rest of those around me began pairing off that it wasn't exactly what I wanted. I've spent a lot of time analyzing it, discussing it, and telling other people about it, but in the end it sort of comes down to that's not the type of relationship that I wanted. The whole "boyfriend/girlfriend" thing exclusivity did not appeal to me, and so I went outside it.
While he wasn't necessarily traditional in his viewpoints on relationships, it was still an adjustment to the idea he was somehow special to me, yet I did sexual things with other people that I also did with him. It's still sometimes a sticking point. What makes us unique and special to each other vs. the other people that we may include in our lives? Like most things in relationships, it winds up being emotional rather than fact or physical. I don't sleep overnight with anybody unless I have run it by him first. That's easy to define, something that I can explain to potential other partners. What gets trickier is the little squishy feeling inside my heart sometimes when I feel his arm around me. That's what makes it special.