I can give the small government answer to this. The concept is that the family should be able to look after itself and it's own members and not the state. Homosexual 'marriage' doesn't do anything that Families in general do - it's not self sustaining, you don't have a second generation - so you end up with two older people without younger folks willing to help in. Who do they turn to? The state.
you know that there are roundabout half a million unadopted children in foster care in america, right?