I have no problem with people being in an open relationship, it's just not for me. My ex had a year to tell me that he wanted to have fun with other guys. But he waited until I completely turned my life around to be with him.
Ouch. Never easy when someone in a relationship makes a revelation that shakes the foundations. We've been lucky to survive a few, but its not something I would wish on anyone. Unfortunately it does happen... a lot.
I know you can't be everything to one person. I just feel if there is a problem, or something missing in a relationship, I want to find the solution with my partner. I don't want to bring in another party to makes things complete.
I think that encapsulates things well by what it misses. Being in an open-relationship, or poly, is NOT about fixing a problem with the existing relationship, or because something is missing in that relationship. Someone who tries to fix a current relationship that way is doing the same thing as the old "we're having a baby to save the marriage" - never works.
Each relationship is a different experience. Each individual human being is unique. In that sense each relationship can bring something to you otherwise missing in your life - but note the wording, I said "missing in your life", not "missing in your other relationship."
As a trivial example, you could have a friend you love to go out clubbing with, and one you love to do sports with. Is something missing from either friendship? Is the night out at the bars less important to you because you play rugby with a different friend?
In open-relationships and polyamory, there is a paradigm shift in how we talk and think about relationships. I'm using trivial examples here, which usually results in accusations that devalue relationships or equate them to friendships - that is far from the truth. I am using trivial examples because loving relationships are for too complicated to summarise in a few words.
I'm a complex person, very different in different contexts, and I guess a bit greedy too. Different relationships let me experience different parts of myself. Its not about what may be "lacking" in one relationship - in fact its not about pre-existing relationships at all! My current desire to date and fall in love is not about my life-partner, it is about me. He just gets the fringe benefits.

