system
Given the following post, respond as if you're commenting on Reddit:
human
I believe post-mortem organ donation should be obligatory whenever possible, CMV

A few things to justify that:

-You are not your body. Your body is a bunch of flesh and bones. What makes you important is that you have a mind and the ability to function. When you die, you are a rotting corpse and nothing more. "You" seizes to exist (and even for religious people, your soul leaves your body, or whatever). So the body of the "you" stops having rights because "you" isn't there.

-It is needed. There are many thousands of preventable deaths in the west from preventable conditions if we had enough organs. The fact that the corpse in the funeral might look a bit bad is not nearly a good enough reason to let a mother of two die because she couldn't get a new liver.

-It really isn't against religion. Most objections for most social changes can many times be traced to religions. However, all the Abrahamic faiths I know of don't consider your body anything more than a vessel for your soul nor am I aware of any modern non-western religion that places any particular importance into bodies.

edit:Apparently, some religions are against it, however most modern, big religions aren't. So eh, half-point there.

So yeah, bodies don't have rights, it is necessary for many people's survival and doesn't have almost any drawbacks that I can see.
gpt
The general reason to oppose it is that when people fear that doctors will not respect their wishes/privacy/etc they are less likely to go to the doctor.  Some people are paranoid that if organ donation is mandatory doctors will not try as hard to save them.  Such a policy would severely compromise their medical care and the medical care of their dependents.  It doesn't matter whether they are wrong or right in their perception.  It matters that they change their behavior because of it.

A compromise position is to make donor-ship a mildly annoying opt-out process instead of an easy opt-in process.  This communicates some social pressure to opt in that increases donor-ship.

In any system, murder victims should never be used as organ donors both to preserve evidence and to remove any perverse incentives there.