We are predisposed to sin. Sin is very enticing because its so much simpler to simply give in. It is due to this that we often choose to sin. I would argue that it is impossible to go through life and not make that choice at least once. That Christ was able to is part of the miracle of His sacrifice, and the very reason he was able to pay all of our ransoms.
A really interesting question is: What would have happened if God did sin?
Ok, then you do believe in original sin, unless you think that Adam and Eve were also (in their original, pre-Fall state) "predisposed to sin".
-A8