Originally Posted by london
What I mean is that we are friends at this point, but we met on a dating site. When we first met and talked about this stuff there wasn't limitations. He didn't say 'we can only ever be "x" and nothing more because my relationship prohibits that'. If we end up staying just as friends, that's fine, but I want that to be because that's what we are compatible for, not because it's against the rules. That's something I feel I checked early on and the only rule stated about his interactions with others revolved around safer sex.
What I do know for sure is that if they closed their relationship, we wouldn't be able to see each other anymore because even though we are friends, we aren't "just friends" and I can't see how we ever would be. Anything that we have exists because they aren't monogamous. But I know she wants to be. And that makes me fearful for what we have.
I understand what you are saying. Your budding relationship was based on one set of circumstances / rules (or lack thereof), and now it appears he may change them due to the other woman's desires.
You should trust what your feelings of insecurity are telling you.
I have been in that exact situation. And the worse did come to pass - the wife changed her mind 180 degrees. Being the wife she had a lot of pull. The question in your situation is how much pull does this woman have? How likely is it that he will cave to her demands?
And I also understand your feeling that if your relationship makes or breaks based on the two of you, you could deal. But having your relationship break due to a coercive influence just sucks.