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.
And that doesn't even get into all the assorted health issues - the fact that 80 percent of gonorrhea cases are 2 percent of the population. It makes even less sense to clamp down on smoking but support homosexuality.
So, infertile people shouldn't be allowed to marry, or people who just flat out don't want to have kids?
