I would say that one non-religious reasoning for opposing SSM is that it is simply not the government's business to get involved in the financial lives of SS couples. If we say that marriage laws and benefits are created with the intention to protect women and married mothers, and in effect protect biological children, then SSM falls outside the scope of marriage laws.
Wow, talk about redefining marriage! How do you think it would go if Ted Cruz or some other 2016 Republican says that SSM should be banned because marriage is only for the purpose of making babies.
Of course this specious argument is just some kind of bizarre legal grasping of straws now that the "gays are icky" and "but the bible says..." arguments are out of favor. However it of course ignores quite of few things:
1. Homosexual couples actually can have biological children (via surrogates and artificial insemination)
2. Opposite sex couples that cannot (or chose not to) have children are not excluded from marriage
3. No state or federal laws distinguishes between biological children and adopted children in terms of taxes, benefits, etc.
I expect that if he or anyone else made this argument he will get laughed at (
just like the lawyer defending CA Prop 8 got literally laughed at when he brought up the procreation issue during oral arguments at the Supreme Court)