Well, it's not just about what laws and institutions will let us get away with. It's also about what we really ought to be doing, what responsibilities we have.
Maybe another way of putting it is this: What puts the "ethical" in "ethical non-monogamy", of which poly is one variant?
As I said, many people here will say "consent! consent!", and that is certainly part of the story. I have responsibility to be autonomous - which includes not being ruled by my own desires - and also to respect and uphold the autonomy of others.
But I don't think that's the whole story.
As an institution, marriage is only partly about the agreement between two people. It's also about contributing to the common good. A nation of stable households might, on the whole, be better off than a nation of free-wheelers doing whatever they consent to do . . . and that being better off may be the sort of thing that matters ethically.
It just makes me wonder, a little, about what we who think of ourselves are really after, and how we should consider our responsibilities.
Is our aim simply to be left alone to do as we choose . . . or do what we want? (Choosing and wanting are not the same thing!) Or are we aiming to transform social institutions at a broader scale, at least to open up degrees of freedom within the institution of the household that have not been seen in our culture before? Do we have to do the latter in order to be able responsibly, ethically to do the former?
"Ignorance more frequently begets confidence than does knowledge" - Charles Darwin
"Mystical explanations are considered deep. The truth is that they are not even superficial." - Friedrich Nietzsche