Interestingly enough, I find myself feeling quite the opposite of you, Ceoli. I want a relationship where I am decidedly a "second" in the hierarchy. That doesn't mean I want to be emotionally marginalized or that I want to be treated unfairly, but I don't want to become a part of a relationship where they want a full time third partner (or 4th partner, or whatever). I am very comfortable in my life being the secondary to someone who has a defined primary.
Just goes to show that there's someone for everyone, right?