There are no intelligent arguments against same-sex marriage.
Not quite, but those arguments are only valid if opposite-sex marriage were legally structured considerably differently from how it currently is structured in the West. I dare say that most opponents of SSM don't care for how OSM is currently done either.
I'd be curious to know what you mean specifically.
Those who believe that the purpose of marriage should be to beget children and arrange for the transfer of dependent females from fathers to husbands of course. Also those who aren't quite so reactionary in their beliefs yet still believe that the husband should be the head of the household and that divorce should be rare rather than having marriage be so readily dissolved.
With the transformation of marriage into a non-permanent legal relationship between two consenting co-equal adults, there are no gender defined roles in marriage law and thus no reason why legally it needs to be one male and one female.