It sounds like you have thought it through about sexual jealousy. I think JRiverMartin is right that you may need to prepare for her loving someone else as well. Sometimes it happens. Maybe you will also find someone you want to be with as well. Once you open it up, it is hard to predict where it may go.
I think a lot of how you should open it up should come from talking to her. See how she really feels about it and be honest about how you really feel. See if this is really an issue with her or not. The success of open marriages is heavily dependent on communication and honesty about your feelings. Both of you would need to feel safe telling the other what they are feeling and work though it.
I wish you luck.