I don't think the 'control' aspect is about 'it's mine and you're not having it'. Besides, I've only seen one person on this thread actually say they wouldn't donate (and they wouldn't receive either).
For me, it's about Government taking control by changing the way consent is given. I'm just concerned that it will set a precedent - ie. your child will be vaccinated unless you opt out - "oh dear, our famously excellent IT skills showed that you hadn't opted out and your childminder, who has bought little Johnny in about his veruccas, allowed the nurse to do the injections. Nevermind eh?" Etc. Etc.
You could argue that no other scenario would be as important as a life-saving organ,and you'd be right, but that doesn't mean that the other scenarios would be unimportant.