If you are married, then so is your wife, and we wonder why she has no "say" in the matter of a swinging lifestyle?? If she is not swinging WITH you, then YOU are not "swinging" either -- you are cheating.
As for your comment that you "just need that little extra excitement once in a while", well... it reminds me of what my mother used to tell me (and, a quote that lives on in our family as the phrase as been uttered to our daughter) - "life is often unfair; you can't always have everything you want."
We have, thus far, only done MFM -- but will never ever ever consider a married man; we are not cheating and don't believe in condoning someone else's cheating behavior. It takes a little more time to find the true single men to play with, as we have to hack our way through the proverbial jungle of "swinging" married men - but we have found single men who we enjoy bringing into our relationship for our play time.