The only (plausible) argument I can see against gay marriage is this. It defies the "original" purpose of marriage to the state. Marriage between a man and a women is likely to produce children, who will in turn contribute to society through paying tax, creating jobs etc. As such, married couples get certain benefits in society which are afforded to them because they will (most likely) fulfill this societal role and procreate. Gay marriages offer nothing to society (in terms of societal benefits) so why should society as a whole be bound to honour them? Gays are afforded all the rights that straight people are given. They can marry any person of the opposite sex to whom they are not related and is above the age of consent. Marriage is not an intrinsic "right" and as such society has placed boundaries on it so that it serves its original purpose. Brothers and sisters aren't allowed to marry, why should this be any different for homosexuals?
This is why this argument is wrong: In today's times, it seems the problem is more about overpopulation than underpopulation. We as a nation are turning back immigrants for fear of over crowded areas and lack of resources. How then can we say that gays can't marry because they are unable to shack up and procreate? Also, I am pretty certain that the modern purpose of marriage is not to produce children. Many heterosexual couples enter marriage with the express desire not to have children. It appears that in today's times, the purpose of marriage is to join together two consenting adults who share love for each other.