From a society's perspective opt-out makes perfect sense. If you are really against organ donation, just register yourself as such; it takes all of five minutes. I don't even care why you would want to keep all your organs after death, it's not my call.
The reason to go for opt-out rather than opt-in is because a lot of people simply don't register their preference, even when they are, rationally speaking, all for organ donation — that's the critical point. Thinking about your own mortality sucks, and visiting the website where you can register yourself as a donor (necessary for the opt-in model) is something that hovers somewhere on the eternal to-do list, but somehow they never get around to it. Because it is never you that suddenly dies in a freak accident.
If you are against organ donation, you will however register that preference the moment such a bill is passed into law.
That's it. Statistics and psychology (and saving lives) are the reason states consider opt-out over opt-in. The state isn't invading your corporeal sovereignty, you are not being harvested for organs the moment you enter a hospital (well, not in France or The Netherlands at least), and you still get to decide.