In our case I have tried being with other women and while enjoyed myself I have decided to remain straight. When I revealed it to my partner he admitted some desire to see what being with another man woukd be like, or sharing one with me. I am fine with either, even turned on by the possibility.
Our relationship has been and still is very stable. I've enjoyed sex with another man and my wife knew about it but she has never had a desire to be with another female. My male encounters started with another male 'accidentally' falling out of my wife and slipping into my mouth while I was licking them together. My wife knew that this happened but didn't care when in other MFM's I would enjoy the other man occasionally. We both are very comfortable with out sexuality and stable marriage. All of the outside encounters for both of us were for the enjoyment and appreciation of the human body that could be had. There were plenty of opportunities for my wife to enjoy another female in the FMF's but she never acted upon it. I respected her decision and she recognized and respected my likes as well.
We have been happily married for 50+ years and still enjoy and respect each other just as much as the day we were married.