Here's my perspective for what it's worth. My partner and I have boundaries in our relationships. One of the main reasons is that we have children together and those children need both of us together to be a support system to them. We can't selfishly become involved in a relationship that becomes more important than the one that we have with each other.
Because of this I tend to seek out others who also have a primary relationship. If it's not a good match for you to be a secondary to someone who has a definite primary it's probably just best to walk away from the situation before getting too emotionally invested.
It's great that you've found someone who is a good match for you.