I suppose a lot of this comes down to if you believe in an afterlife or not and such.
I would rather it was an "opt out" system where you have to go out your way to opt out, otherwise your organs are fair game to save other lives.
Its simple in my eye, when you kick the bucket, your not gonna care or not if your missing a few organs, why deprive someone else of an extended life by "taking it with you" when in a few hours time it is already worthless and in a months time its gonna just rot into the soil and degrade to nothing anyway.
Frankly, I think it is selfish of people to "not" want others to have an extended life by donating organs of people who have died. Ok your loved one may have died, but why should another person / family suffer because you dont want them to use the replacements?
Alternatively look at it like this, would you take an organ to save your life if needed? If yes, why arn't you a registered donor if you haven't signed up already?