...Redpepper's right, but we can offer thoughts on your particular situation too. Why not?
You're not alone: the difficulty of seeing a spouse or lover forming an emotional connection with someone else seems to be, for many people, more threatening than a mostly sexual connection. There was a forum topic over on OKC just a few days ago in which a man asked why he could accept his wife having sex with a BF but not her sleeping over with him. The consensus seemed to be similar: it is the emotional significance of them spending the night and waking up together that he found difficult.
Personally, I think that part of what drives monogamy is a need to be the one special person in someone's life. So maybe if one's emotional self identifies sex as defining that specialness then one will feel sexual possessiveness, and if one identifies emotional closeness -- love -- as that specialness then one will feel emotional possessiveness (for want of a better term).
In my own case I seem to be able to accept that I'm special to my GF even though she also has another very special person -- her husband -- and may have other lovers at other times. I might feel differently if she and I were in a primary relationship; I haven't tested myself in that situation yet. I hope I would accept whatever she needs for her happiness and I kind of suspect that capacity is there.
But here's a question: assuming you might want to change the way your emotional self looks at its own need for specialness and relationships, how would you go about that? I rather suspect that these things operate at a subconscious level. I suspect one doesn't simply say, "OH, right, it's illogical to feel this way so of course I won't feel it anymore."
I do think people can change. Hell, I know it because I have changed. But techniques for changing and the time needed to do it are another matter.
Does any of this make sense? Or am I wandering off base?