It might be that instead of a committed poly type relationship what you are really looking for is a shorter less intense encounter like a swinging type relationship. Sigel is like you, XzombehxbearzX, in that he loves me with every ounce of his being but still likes the amazing feelings he gets meeting and enjoying a sexual encounter with another person (or even couple). He doesn't want to invest a lot of emotional energy in another relationship (though he is open to the idea should the right woman come along).
I am not the type to desire casual sex or non-emotionally invested sex as I don't really get turned on by the pysical sensations of sex as much as the stimuation of being with someone I care for deeply. So for most of our marriage we had a series of poly and swinging type encounters that went horribly wrong.
Once we learned to allow each other to decide for ourselves what we wanted/needed from our relationships we really connected as a couple and were able to really appreciate being together but open.
So all that being said, I would give you the same advice I gave Sigel and Arch gave us both: Do some soul searching and really decide what, exactly, you are looking for. Once you can communicate what it is you are wanting/needing you can begin to ask for it...until you do this youare shooting in the dark hoping to hit the target, which isn't fair to either of you. Your parter needs to do the same for herself. This isn't a cooling off period, mind you, just a time of reflection and inward looking.
Use the time to REALLY get to know your partner and yourself and how the two interconnect..this will have two purposes:
First it will crystallize where your lives intersect relieving jealousy and possessiveness (if you already possess it you won't NEED to posess it). This allows for a playmate (or even a potential third partner) to have a special place that belongs to them woven into the web of your lives more naturally.
Second it allows you to go looking for what you really want/need in a playmate or partner. This sets you up for success rather than another failure. Even if you never find anyone else, it opens the door to the possibility, which, I am sure you will agree, is the really exciting part of being in an open relationship!
On another level this sort of deep reflection and connection also allows your lady to feel like she is part of this whole adventure rather than the one on the sidelines. She will be better able to advise you and care for this potential playmate/partner because she will know the reasons you are attractd to her/him. This also alleviates jealousy or at least mitigates it.
Anyhow, good luck and keep on lovin'