Why are you so opposed to a life-long relationship? I think this is the big question here? While I wouldn't go into a relationship with it having to be life-long, if someone was completely against the idea from the outset how could I open myself up emotionally to that person knowing that I had a finite time to be involved and then he would be moving on?
If after a certain period of time being in a relationship one party is wanting serious commitment and the other isn't that's when you address the issue. I don't think you can definitively know how you will feel after giving yourself honestly and lovingly in a relationship.
If all you want is a casual relationship (or relationships) that's a different matter but I would have thought there would be others out there looking for something similar.