Keith says:

"Society's mores are always but slowly changing.
They never have and never will change for everyone on the same day."

But embedded in that seems to be the idea that laws allowing those whose mores HAVE changed to live their lives as they wish should not change until the rest of the slow ones catch up!

"marriage is much older than Christianity, and it has always been between man and woman until now."

   No, it has NOT 'always' been any such thing. It has usually been controlled by churches... who put their stamp on it.... which served to fairly effectively keep gay & lesbian couples from attempting open marriage.
   Simple descriptions of unfair practices of the past are being used to justify them!
Countries, along with 'approved' churches, used to allow slavery, engage in wars of conquest, execute 'heretics', extort 'taxes' for the sole benefit of the rich, burn books that they did not like, refuse voting rights to women and other groups and imprison or execute people for minor offenses. These practices were wrong, even though they were common and 'slow to change'... and denying marriage rights has always been wrong, no matter who once controlled the process and pretended that 'control' constituted a 'definition'.
The word signifies, as Muskets notes, a bond... not a limitation.