Okay, couple of things:
*Concerning the whole "My things, only I get to decide" line of argumentation: It doesn't work like that. It really doesn't. Invalidated by one word: Taxes.
So, we have our precedent. As long as You are in a state which taxes its citizens (aka. pretty much every state), You're already admitting You don't have full control over Your own ressources.
You can argue that You don't want to be taxed, too. It's just that arguments like that tend to be smashed in a more or less serious debate pretty quickly. There are a lot of laws passed over "I don't like it", true, but it's not exactly a desirable state of things - except for people who happen to like or dislike the "right" things.
*Second, Rights. That's a whole bag'o'worms here, because it's difficult to find an universally acceptable definition. They can be conventional, arbitrary, god-given or nonexistant, depending on perspective. But they're not set in stone, as historical precedent proves. One does not even have to go far back to find it, just think Woman Rights, or Slavery in the US.
So what opponents of an opt-out system say is: It is my right to decide what happens with my body after my death, and I want it to stay that ways. Well, we propose to narrow that right down. Why not? We do it in other areas as well. It's not even that we want this right totally gone, which wouldn't even be that unreasonable - just narrow it down. And we back it up with arguments other than "Because we feel that would be great!"
As for another precedent for that - mandatory schooling. Most people agree that the raising of a child is a parent's right, right? But if You arbitrarily decide Your kid doesn't need to go to school - nope. If You decide it's okay to give Your kid some practical sex ed lessons - nope. So there's that.
Ah, but that's a conflict case! MY right to parenting as I see it fit conflicts with my child's right to have an education and not be molested!
...exactly. My right to live trumps Your right not to feel uncomfortable for a while with the thought of Your body being harvested after Your death - or not to have to waste time to fill out a form. I'd say that's reasonable.
*Third, yes, I get it, humans will be viewed as nothing more than a meat bag with spare parts in it! Except, not really. Yes, this MIGHT happen, and that would be a perversion of the rights we propose and a totally different thing altogether. Saying an opt-out system would lead to state-sanctioned murder and body farms is like saying allowing gay people to marry will lead to mandatory anal rape every friday. In other words, does not follow.
Besides, if anything, it would help to reduce the risk of that happening. It's not like spare kidneys would suddenly become a consumption product (new kidney every week!) A good comparison is the abortion debate, though let's drop the question of morality for a while: Legalizing abortion does not mean it suddenly becomes a fad. Abortion is a very complicated, risky procedure, and so are organ transplants, just much more so. No-one really wants to go through it, not even once, so it's not like the demand would suddenly skyrocket. The demand is here, and it's high - it's just the supply that would be equated.
Yes, there might be some cases where people will get careless because the option of abortion exists. Likewise, I don't deny that there is a possibility of some rich guy identifying You as a possible heart donor for his lovely daughter and killing You for that. Except that people inclined to do that and who have the ressources to do it can do it anyway, right now. Because we're not proposing to murder people for body parts, that would still be as illegal as it is now - if not more so.
*And finally, arguments from the individual. I know, it sucks when YOU are the one guy who gets shafted by the new system, but that's the thing about considering new laws and altering existing rights: Someone always suffers. The trick is to minimize the suffering. And an opt-out system is the best thing to do that in this case. We have three groups: People who want to donate, people who are dead against it, and people who don't care. Let's assume that the distribution is equal for a while. Current system, 1/3 of possible ressources can be utilized, and, barring error margins, everyone is happy. New system, 2/3 of possible ressources can be utilized, and everyone is STILL happy - again, not counting error margins. But You always have error margins. Idealism is awesome, and I'm quite an idealist myself at times, but it's just not very practical.
Considering the people in the error margin here are, by definition, dead at the time of harvest, I really think there's the least harm done like that.
~Sylv