NC, if you don't mind, I'd like to add a few things. . .
Hubby and I have been together for over 10 years. Our first encounter with another lady didn't occur until about our 8th year. The first time was with a girlfriend of mine. It lasted about 3 months before she wound up telling him she was "in love with him". That was the end of that.
I think you have to draw a fine line between being emotionally attached to the X-tra lady and having a little fun. I don't think it's fair to anyone involved if the lady is just picked at random, there will be no emotional connection and I find that leaves the sexual experience flat. On the other hand, if you do connect emotionally with the X-tra lady, you will find that it gets more difficult as time goes on. Granted, the sex gets hotter than hell but the emotions grow deeper.
The hardest part for me was when we had finished "playing" and my girlfriend had to leave. I felt guilty that she was going home to an empty apartment while I got to stay with "the man". There was always that awkward moment at the end, no matter how lovingly we said good-bye.
In the end, as much as I was willing to share his body with her, I refused to share his heart and the friendship ended with her revelation to him.
Like I said, a fine line.
Good luck to both of you. You sound like you have your head in the right place and your heart in your wife's hands. ::soft smile::