You can't put the genie back in the bottle, so this might be too little, too late, but I made the decision the other night (before reading this post) that I'm going to stop trying to take this situation out of fantasy land and into real life. My wife and I have incorporated this fantasy into our private sex life for about 2 years now, and like you it started off with her doing it to turn me on. It progressed to where she began enjoying it, and then it got really fun. Now it's not completely restricted to the bedroom. On the few occasions when we go out to bars she will wear a low cut top and a push-up bra. She has large C cups so sporting great cleavage is not a problem for her. Guys stare and we both get off on it, and once in a while if she sees a guy that she thinks is cute she'll flirt with him a little, but that is rare. So here is what happened to us the other night...
We had a chance to get busy, and decided to have a few beers and relax first. I had decided that I wasn't going to bring up other guys, but surprisingly SHE did this time. I figured I'd go with it, and after a few more beers she asked if i was serious about her seeing other men. Thinking we were still in fantasy land, I said "Sure, go for it". We talked a little bit more about it and she pushed the envelope more and more about having sex with the guys she dated and so on. Still thinking we were just talking junk, I continued on. Well, we got to a point where we'd both had a few beers to many and then she asked me if i was being 100% serious because if I was she'd start dating right away. In my takesen haze I realized she was really, REALLY being serious, and I told her that as long as she didn't leave me I'd love to see her fuck another guy. Well that was the wrong thing to say at the wrong time. She started going off about how i really didn't love her and if i did i wouldn't give her pussy to another guy and blah, blah, blah.
Too much holy water and too many crossed signals lead to the biggest arguement we've had in years, and both of our plans to fuck like rabbits turned into door slamming and me relaxing on the couch. Lesmister learned on my part. She loves the fantasy, but is never going to take it any farther than that. I've decided that I'll never bring it up to her again. If she wants to play the game in the bedroom then she can bring it up, and if she wants to flirt with a guy to turn me on then she'll make that decision.
I'll still fantasize about it, and still do Google video searches for "wife with BBC", but that's all. It became too consuming on my part, and too confusing on hers. see, as long as it remains a bedroom fantasy you can dismiss it later as just some kinky fun and move on, but once another guy puts his dick in your wife, you can never go back and you can't pretend it didn't happen.