OP, I get what you mean about soulmates. The typical idea of soulmates is monogamist by nature because it requires there be one person for every one person. But I like redpepper's tweaking of that theory, and I find it similar to my experience, too.
Not to thread hijack, but one thing that has been interesting about meeting my OSO was that the feeling I had when we met, and immediately after, was extremely similar to the feeling I had about my husband when we met. It's the feeling of "I was meant to meet this person, meant to have a relationship of some kind with this person, and this person is going to have a profound impact on my life--I just know it to the core of my being."
I've had that several times during the course of my life, with different people, and every time so far I have been right. Interestingly, some of them I had long sexual relationships with, some short, and some no sexual relationship at all...but they all had a profound impact on the way I look at the world, at a time when I really needed a new perspective