What other things that you can do to yourself are illegal? Suicide. Abortion in some places. What if I cut a piece of my body and eat it, is that cannibalism?
I think at that point, you're likely to be involuntarily committed anyway :)
> one has committed a crime or not depends entirely on the other party will decide at some point in the future
That is already true. In many crimes you need to press charges, and sometimes you can even request the charges to be dropped.
Think about a simple case: a coworker slaps you. Is that an assault? Depends on the exact details and on your relationship with him.
But hey, having fun is immoral.