As a couple who has apparently quit, I would venture to say the the lifestyle quit us...it wasn't a conscious decision. Over the years that we were actively involved, our favourite clubs closed and our couple friends split up (all of them). Aside from getting a "black widow" complex, it became discouraging to meet new people. The thought of meet new couples became tiresome and the overall appeal was reduced to a chore. Thats not to say that presented with the opportunity it would not be seized, just not sought after.
I do not believe that if the lifestyle was a positive experience, you ever leave it. The evolution and thought processes that were experienced along the way become a part of who you are and in turn I believe change your outlook on many things.
Annette