Ok, here are some of my thoughts on the matter...
1. I have to agree with what XYZ said a soul mate is, someone who touches your life and makes you feel complete in some way and also with Mono about how you feel like you have always known and loved this person. To me, a soul mate is a person so special that you cannot imagine your life without them in it.
2. I think being poly has opened my definition of soul mate as it has relationships. For a long time I believed in the fairy tale version of soul mate, that one person you are meant to be married to...kinda like what Quath was saying.
3. I don't think we are limited to only one soul mate. I am reminded of a scene in Ever After where the Prince is talking to Leonardo and asking him about his "match". What if's abound from, what if she is struck by lightning, is that the only chance at happiness or could there be a second? What if they were both walking side by side and you just happen to meet one before the other...etc. So if we are not limited to the number of soul mates we may find in life, that leads me to the answer for #4
4. No, soul mates do not have to be romantic or sexual. Again the fairy tale idea is that soul mates are supposed to be marriage mates as well. Maybe that is the case but if you meet two different soul mates and end up in a relationship with one (say you are mono) does that mean you have to completely ignore the relationship with the second? I feel that a person can be a platonic soul mate, although sometimes I wonder if it is supposed to be platonic or if it is made/chosen to be platonic due to circumstances.
I'm sure some of you may see where this is coming from and going, perceptive people that you are.
I believe that I have found two soul mates, one that I am married to and one that will have to remain "just friends" but although I love them both in two completely different ways romantically, I do not want to imagine my life without either of them in it.
So there is my $.02 for now. This can always change and morph into other ideas.
So anyone else care to contribute their ideas?