Who said anything about denying monogamous couples anything (other than in jest)? We're talking about allowing poly folk the SAME advantages that "traditional" relationships enjoy.
For instance, if Lana stays with us and "marries" us in the current political and legal landscape, it would be a strictly ceremonial thing, not legally recognized. While that's fine in many ways, she could never file as "married" for taxes, couldn't get family health benefits, etc on so on and so forth.
Same thing gays have been after for so long. Don't deny anyone what they have now, but why does the legal system get to sy that because two people are the same gender, they can't have those same legaal benefits? Or because 3 or 4 people choose to intermarry, only 2 of them get to have it legally recognized?
And don't start the old "people would abuse it" BS. Mail order brides and a dozen other forms of marriage for convenience are already long standing "traditions", lol.