While I disagree with them, one can argue that since marriage under American (and other modern legal systems) is NOT concerned with “intimate” behavior (due to the decriminalization of various crimes such as “adultery”, “sodomy” and “fornication” – all of which were involved in criminal prosecutions of unmarried people until the mid-20th century), what the goyim call marriage is solely a contractual economic relationship, and there is nothing in halacha that prohibits two persons of the same gender from entering into an contract to combine their assets and live together as a single household (and such is often done in our community, such as when unmarried adult children move in with their widowed parent, not to mention two singles becoming housemates). Since “marriage” in their system is solely about economics, and since the prohibited behaviors are now lawful under their law regardless of marital status, there isn’t an halachic problem.
There is a practical problem that by redefining marriage to include many childless couples, there is a liklihood of losing many of the protections that existed when marriage was primarily designed to protect dependent mothers and children, since same gender couples (who rarely have children) and the many married childless couples, do not need such protection.