The argument will be very simple and concise:
1. Do boys need a father and girls a mother or at least the hope of having one (gay marriage strips them of this hope even for life)?
2. If #1 is "yes, or they will statistically suffer a number of social and psychological maladies", then New York vs Ferber will be cited to show that even if the Judicial branch is allowed to add brand new categories to the 14th Amendment for protection (just some of their favorite repugnant minority deviant sex behaviors-as-identity but not others), those constitutional protections do not dominate a child's right to hope or presence of the missing gender-as-role-model for a parent.
3. From there (or even before, forcing the issue) states can say "we have no reason to give monetary incentives to what amounts to a mental prison for children...a prison from which they will arrive into our social milieu as predictably stunted and struggling individuals headed for indigency, mental health issues, drug addiction or prison. ie: States cannot be required to invest in their own predictable fiscal demise.
PRINCE'S TRUST 2010 YOUTH INDEX SURVEY