Do you not understand the most basic concept of organ donation? Donated organs save people's lives. If you choose to opt out then you are actively choosing to let someone die because you think the law is 'presumptuous'.
I hate this argument.
Every time you spend your money on a night out/chocolate/luxuries, you are choosing not to give it to charity, so are actively making lives worse for selfish reasons.
That's right, it's your money, so if you donate it to a charity, that's great, it's a gift, if you choose to keep it for whatever purpose you choose, that's fine too, it's yours to do with as you please, whatever the 'rights' and 'wrongs' are.