I'll will try to formulate an argument in opposition to gay marriage. I do believe it should be allowed, because it's not my business who's married to who, but for the sake of argument:
The problem with gay marriage is that it is not overwhelmingly accepted by society yet. There is indeed a majority in the society that do, but I believe the percentage is at 54% to 42%, hardly overwhelming.
Ordinarily, this would not be a problem. But marriage often has this next step of having kids. Obviously, a homosexual couple can't have kids so they will have to adopt. The problem is, when 42% of people, a significant part of the population, are against gay marriage, it may become difficult for the son or daughter of a gay couple to fit in. We don't want such discrimination to occur, so why not wait till gay marriage garners more overwhelming support?