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.
Its silly to say that a free society must be rejected because it is free to be wrong. Freedom only exists where you can do stupid or wrong things.
You are part of society so its not possible to reject it. You may loathe it but the very act of communicating that fact makes you a part of the society. There is no point pretending that society doesn't exist or that its action are illegitimate. If there is something you disagree with, you can set about changing it. But you have to start from the reality that society exists and that it can legitimately try to improve life for its members.