Once it gets to the point where it is self directing via democracy, it can legitimately act on behalf of the plurality of its members.
Why?
You ask Is society entitled to declare slavery legal? It already has for 1000s of years.
That was my point. It shouldn't be entitled to do that. Since you think it does have that entitlement, since you think it is entitled to declare anything it damn well pleases, I must reject your beliefs outright. I consider your position reduced to absurdity.