If gay marriage becomes a reality, the meaning of marriage will change. Marriage will no longer be used as the name for that ancient institution between a man and a woman.
For a secular person the main reason to defend traditional marriage is to defend something like an ancient "trademark" or "copyright." Marriage was a word given meaning by heterosexual men and women who entered the traditional union between a man and woman for the purposes of resource sharing, breeding, etc. Gay people didn't have anything to do with it. They haven't been the ones keeping it alive throughout the centuries.
If the activists for gay marriage manage to redefine the secular meaning of the word, religions of the world and people who practice true marriage can stop using the word marriage altogether. Marriage would then just come to mean civil union, no?
At the end of the day, a gay marriage will never be the same as the ancient institution between a man and woman.
What do you guys think?