Ignoring the symbolic aspects, most governments have adopted marriage as a bureaucratic detail that determines access to a variety of rights, priveleges, protec

Ignoring the symbolic aspects, most governments have adopted marriage as a bureaucratic detail that determines access to a variety of rights, priveleges, protections, and services. People who have established a stable family unit that looks like a marriage in all ways ways besides the gender of its members naturally want access to those same rights, priveleges, protections, and services. In most cases, because of a variety of official or de facto details, a "civil union" or "domestic partnership" is not functionally identical to a marriage as far as this goes and so generally doesn't make for a convincing compromise.