Well, I think it's perfectly normal to feel jealousy...what I think matters is how you handle it, not that you feel it. It also sounds like the jealousy is over fear of an emotional attachment versus fear of physical enjoyment...I think that's understandable too.
I know from myself, you probably have some kind of fear or insecurity at the root of your feelings of jealousy. Do some introspection ("Why do I feel jealous about this? What is it I'm afraid of?"), and talk with your wife about it. Figure out why you feel that way, and then you'll know whether it's something that you or she can remove or not, or whether you guys need to adapt how you handle the communication to not trigger jealousy for you. I think the fact that she said it's OK for you to play around, but she doesn't want you to fall in-love and leave her should be a really, really reassuring statement to you. She doesn't want to lose you...so that certainly suggests that she's not going anywhere either!
I dislike the green-eyed monster myself...I don't like to feel jealousy nor make anyone else (especially my wife) feel jealousy. I don't feel jealousy very often, but when I do I usually work it out for myself. If I can't work it out, I talk to my wife about it, but in a very non-accusing way. I completely trust my wife, and I have never had any reason not to. Even if something doesn't "look quite right" to me, I assume she's innocent rather than guilty, and just talk to her about it. My wife has come a long, long way in overcoming jealousy herself, and the good effect for me is that she makes me feel very trusted. That's good! Relationships have to be built off trust, or they probably won't last.
In the big picture, I just don't think there's any truly valid reason to feel jealousy. If someone is going to leave for someone else, would you want them to stay with you anyway? I don't like to chain or be chained with jealousy...it's VERY reassuring to set someone "free" and watch them NOT leave. You know for certain they really WANT to be with you (and you wouldn't know that if you chained them to you with jealousy...with me?).
Trust breeds trust, and distrust breeds distrust. Even though you have some of these feelings, be sure to keep it stacked on the trust side rather than the distrust side.
Loki