That's a great theory in principle, but your own comment reveals reality is more complicated. Pedophilia doesn't inherently hurt anyone by itself, and making it

That's a great theory in principle, but your own comment reveals reality is more complicated. Pedophilia doesn't inherently hurt anyone by itself, and making it illegal doesn't make it go away. And most people do believe that homosexuality causes a net harm to society (probably falsely, but still.) Or that marijuana is a net harm, etc. And even if we establish what does and doesn't have externalities, the idea that they should be legal isn't universally agreed on, obviously. I mean heroin addiction causes a lot of harm and is probably a net negative utility to users. Even if it's voluntary, the world would probably be better off without it.