Because there are people with behavior that do not conform to the agreed upon norms.
My ethics? I never said mine.
And yes, there are inherent dangers (like tyranny of the majority), but there are trivial ethical issues where there is no good solution for allowing the [or any] minority to self-determine. (Like rules regarding sex with children, just to throw in the classic think of the children. But of course the age of consent is not a magical line somehow creating an overnight transformative event for every resident of that state.)
If not, how do you know it is in fact the right thing?
If you prescribe and enforce your ethics, I think you're wandering close to another definition of politics.