The laws against murder, theft, drunk driving, etc, etc, and the laws already on the books regarding obscenity, child pornography, etc. are ALL about morality. Do you seriously wish to posit the argument that they should be eliminated? I would say that all those laws were enacted not because G-d said they were bad, but because you cannot have a society grow and prosper without them. And that those that submitted and passed them did not do it for the sake of the Lord.
That may well be, but it doesn't change the fact that they are all laws that 'legislate morality.' If we did not have laws to do that, there would be no laws against murder or a father having sex with his daughter.
The leftist (false) argument that we should not 'legislate morality' is not based in the reality of what the laws we almost all agree on are there for.