"Societal harm" sounds collectivist to me - can you be less vague about the nature of this supposed harm?
Would you have prostitution de-criminalized?
It seems to be working well in Nevada.
All drugs?
The only demonstrable result of drug criminalization is to enrich criminals ... although that said, if we'd abandon the rank nonsense of banning pot we might find ourselves left with a war on drugs we can actually win.
Polygamy?
If you mean polygamous "marriage," as I said about homosexual "marriage" it's another thing entirely, since marriage is a positive act by an institution (government or house of worship).
Public nudity and obscenity?
No, I didn't consent to accept the light reflecting off from those acts.
Just as I thought. You are no different than a liberal. Your purported reasons are just different.