It seems that this all boils down to the GoldenRule: Do as you would have done unto you. Most of the above has come from the side of those that are not on the receiving end of this. It is extremely painful to find out someone that you are involved with has placed someone else, even momentarily, in your place, and then still expects you to remain fainfthfully there for them. It's selfish. Yes, sex without commitment is great if that is where your values are, but then at least have the courtesy to be involved only with those of the same values. You just can't do this to someone you care about and expect them to accept it. Your need has now taken precedence over what you both have together. Monogmany is important to some people. I am one of them. It is a strength that binds, and more importantly provides peace and security in world that is so difficult. Not to have to worry about the one you are involved with, having that security, that commitment, makes life so much more wonderful and fulfilling. Simply, would you want someone to treat you this way? Having been on the other side, I can tell you, it hurts more than you know. No one deserves that for someone else's selfishness.
Stronzo, you have something great. To have a relationship that has lasted as long as yours is something to be held sacred. Don't take for granted what you have. Work out whatever you have to work out and don't throw away this wonderful bond over something meaningless. I wish you both happiness.