So your argument is that marriage has no purpose. Then we should eliminate it not expand it.Since you obviously think same-sex marriage is NOT good for society, where's your support for that contention?
Let's me shoot down one obvious argument you'll try to make:
Raising children? Nope. Tax deductions for dependents are available to both married and unmarried parents. And more than 50% of children in America are born out of wedlock. Furthermore, lots of married heterosexual couples have no children and many are incapable of having children (yet we allow such couples to marry). Finally, many, many gays are capable of having and raising children (sperm banks, surrogate mothers, adoptions). So it's rather difficult to argue that there's any child-raising benefit to being married, or any benefit that's exclusive to heterosexuals that wouldn't also accrue to gays.
So explain to us how heterosexual marriages are "good," yet same-sex marriages would not be "good" in exactly the same way?
EDIT: And it is bad for society because it will decrease tax revenues and increase burdens on divorce courts.