Well, this was predictable. A group of three American women have "married" each other and are now expecting a baby.
I don't know on what basis liberal moderns can object to this. If marriage is understood to be a public commitment ceremony only, then any conceivable configuration of marriage can take place (why not three men?).
The alternative (traditional) understanding sees marriage as a uniting of distinct and complementary masculine and feminine persons, as an embodiment of faithful love, and for the lifelong purposes of conceiving and raising children and maintaining an intergenerational link of family.
It is from this particular understanding of marriage that marriage becomes something that involves two persons only; that it becomes exclusive; and that it has a lifelong character.
You can't take away the traditional understanding and then expect marriage to retain the same characteristics that it once had.