Far as I can tell there are no valid legal arguments against it [same sex marriage], save for that which delusion allows. But then, reality sucks, doesn't it?
That'd be like an officer of the court arguing that ugly people shouldn't be married.
Or, more appropriately, interracial couples.
As NefariousNewt points out, it is homophobia. There really isn't anything else, again, save for what delusion allows. How one could argue for this kind of banning is beyond me. It'd be really interesting to see someone even attempt to conjure up any kind of statistic that could support it.