That is quite subjective.
Some believe that people are genuinely harmed by taking part in sin, while others don't believe in sin.
Some believe that people engaging in unnecessarily risky behaviors (e.g. homosexual sex) are harming themselves. Others think it is perfectly okay as long as they are only harming themselves.
Some believe that allowing people to engage in perversion affects all of society, so such behavior should not be permitted. Other people believe that what one individual does to themselves (or with a consenting partner) has no impact on society.
So you see, your 'objective' criteria is not so objective.
Fine, then.
*Demonstrably* harmed by the activity. Not having their delicate sensibilities "offended" (which is nothing more than social conservative Political Correctness), but demonstrably harmed.
Why do you want to give the government so much power? When you're asking them to patrol men's hearts, that's exactly what you're doing. Myself, I don't trust the government enough to advocate that.