You bring up a very good point. I (husband) would no doubt be happy with my wife if she said she didn't want to be in the lifestyle. But I admit I would definitely feel like something was missing. Having said that I don't know if that would lead to some animosity later on.
My view of having sex with others is it's just that SEX! Now of course that view is from a male mind and I know women (according to my wife) typically like to have an emotional bond with those they have sex. You can have sex with anyone but love doesn't work that way.
I couldn't agree more with those who say how liberating it is. My opinion is that we are not wired to be with the same person sexually for our entire lives. My wife said fucking other people in her previous marriage is something that she would have never considered. Her views, of course, have changed since. The first couple of times she said she felt a little bit of guilt afterward because she felt like she was committing adultery even though I was there. Has something to do with that good Hispanic catholic upbringing.
For me the bottom line is if you have a good solid marriage prior to entering into the lifestyle I believe the marriage with be fine. So for those who want to give it a try do it. If after the first time either one of you don't like it then don't continue. But my bet is you will continue. It's been almost nine years since her first time with another man and now she "loves it." Can't wait til tomorrow. This is who she be with then.