I was in a committed lesbian relationship for six years, before falling in love with someone else. It was not that I didn't not love my girlfriend any less or she wasn't "meeting all my needs". We were the poster children for the perfect lesbian couple, no jealously, no fights, everything was harmonious. Someone just came in to my life that filled a different part of my life, that I never knew was missing.
Dealing with the conversation of a poly lifestyle was hard, but in the end the six year relationship is actually stronger because we are in it by choice. It is not for everyone, but you definitely have to have a stable base and each of the participants have to have rules and lack jealously.