Really interesting subject!
I actually think of it all as a jigsaw puzzle. Humans didn't evolve to be "alone". We're actually quite a clannish species... so when we form a group, we create a "picture", and each piece has its own size, shape, and numbers of connecting arms.
For example, take two mothers, and we're looking at the "progeny" part of their pictures. One mother has four children. The other mother has one child. Are their "jigsaw pieces" the exact same? No. The mother with more children has more connecting arms on her piece in order to link with all her kids. The mother with just the one kid has just one connecting arm for the "progeny" part of her overall picture.
I consider it the same for mates. A mono pair would have just one arm each, for the "mate" part of the picture. A poly person would have more than one, at least.
And I don't think it's a static sort of thing. I think the puzzle is very dynamic, and each piece can reshape itself to adapt to the situation as needed in order to create the whole picture.
So in short, I think of "soulmates" in terms of... not being fated, but something that we each create, when the time comes for us to create it.
Does that make ANY sense? I'm hungry so I may not be as coherent as I think!