I had to point out that the anti-gay marriage people believe that marriage is between one man and one woman. That is fine, but that means that if the one man and one woman get divorced that is not the end of their marriage to one man or one woman. In their vows they promised among other things to love, honor, etc., until death do us part.
If they were to be remarried then it would be more than one man and one woman. So I cannot see what the problem is with gay marriage. I have several friends who are gay and they have been together longer than some people that I know who are straight. Myself, I have been happily married to one woman for almost 29 years.