This seems to me to be the crux of the biscuit, STS. When I was married, I never once felt in the least bit threatened, swayed, influenced or otherwise effected by same-sex couples wanting to marry, or at least have a civil ceremony.
Hetero marriage has been around for eons. This little blip on the radar won't do a thing to change it, and it's at least a little disengenuous, IMHO, to maintain otherwise.
Well, it will turn it upside down, overthrowing thousands of years worth of tradition. But (seriously) how do we know that's bad? I would say that it is very bad indeed, and here's why: It completely changes the financial and child-rearing basis for society. It's a giant leap.
As others have pointed out, you can have a tax return with 50 spouse deductions. You can avoid inheritance taxes by marrying your grand-daughter. Is the government going to like having these financial rules changed?
How about children? Homosexual activists like to pretend otherwise but the statistics are clear: homosexual relationships do not last very long (VT is already dealing with gay divorces), and partner-abuse is higher in gay relationships than in hetero relationships, and child molestation is higher among gays than straights. Children lose big-time if their parents are gay.
So, we overturn one of core religious tenets holding our society together, and in return we get financial chaos and more disrupted childhoods. Great deal.