I don't have a problem with that. People in polyamorous relationships should absolutely be drawing up contracts, powers of attorney, agreements on child custody, etc etc etc.
The issue of course in calling it "marriage" is that things like benefits, social security survivorship, tax stuff, etc are predicated on two people. I just don't think there are enough of these relationships to make us change all the 2-person-relationship laws (though others may disagree, of course).
But yes- if you are doing this - and many are (5% according to one article I glanced at briefly) I hope you are protecting yourself and others through such contracts and such.
And maybe two person relationships should be governed by contracts as well! Might spell things out a heckuva lot more than they are currently!