I think it would be immoral to opt out. Donating your corpse's organs does you no harm, and it benefits others. You don't get many surer chances to improve the world than that. One might feel a bit squeamish at the idea, but that's simply irrational. In any case, surely one's discomfort is not more important than another person's life. To opt out of organ-donation would thus be an act of extraordinary selfishness.
Despite this, we might think that individuals should have the (legal) right to dispose of their body as they please. For example, it could be argued that to grant the state such powers of bodily violation would set too dangerous a precedent. But there seems a clear enough line between living and dead bodies to avoid any "slippery slope" here.
Alternatively, one might point out that the policy is disrespectful of
A tempting solution would be to offer an inconvenient opt-out option. It should be enough to deter those who are only mildly opposed to saving another's life. But the option is there and attainable (after some inconvenience) for those who feel very strongly about the issue.
A more principled liberal option would be to promote individual choice (rather than the common good) by ensuring that everyone can easily opt out if they so prefer. Thinking that it's not the government's role to place hurdles down the road of vice, we might instead rely on social norms to discourage people from making the wrong decision. People tend to go along with what's expected of them anyway. If organ-donation was widely accepted as the norm, squeamishness or opposition to the idea might disappear almost entirely.
So: which would be best? (Or are there other options that I've missed?)