Organ donor should be opt out, not opt in.
I think it's the right default, most people won't mind donating and those who do are generally more motivated to fill out the opt out docs.
That's a contradiction. If it's mandatory, it's not a choice.
What makes you feel you have the right to tell others they should be forced to donate their organs?
Where I live, one must state organ donor preference when obtaining a driver's license. I believe the default answer is no if you choose not to choose.