Is it wrong to legislate morality at all?
You should do no harm to others IS a moral statement. But, is this the only morality a legislature can dictate?
While I generally agree with moral liberalism personally, I've got to say that philosophically it's really silly to think legislatures can't legislate morality. I think they can -- so long as they do it equally for all and do not otherwise violate the constitution.
Which leads me to note: legislating morality and establishing a religion are not one in the same.
As for the argument about polygamy, Maelakin answered it best, absolutely more sensible than my reasoning, which was a collage of legal research I did several years ago.