Because people don’t agree on what we “ought” to do.
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People don’t need to agree on what is right and wrong to make a law in our system. It’s a majority rule system with some rights guaranteed in the Constitution. All else is a matter of persuasion and numbers. Shorhand, but you get my point.
I think it is much, much more important to fight for the right to do things we “ought” to do, for example speak freely, than for things we “ought not” do, such as prostitution or drug use.
And, does your position imply that what every is not expressly permitted is forbidden? What about things that are not "oughts" but not forbidden? What about superogatory acts? The difficulties in your proposal are legion. It's just not worth talking about any further.