I think some issues are being mixed up.
The people pushing gay marriage have made perfectly clear they do not consider a civil union good enough. So whether or not they function well is actually a red herring. They want a full blown removal of the gender restrictions on marriage. They are claiming that ability to reproduce is suddenly irrelevant to marriage.
That leads me to wonder if this opens the door to abolishing other gender restrictions. Let's say you find someone who claims, like gay marriage supporters, that separate but equal restrooms are inherently discriminatory because they stigmatize all men as pervs or women as weaklings. It could be argued that segregation, regardless of the justifications offered, is irrelevant to going to the bathroom.
You just dismiss this all as absurd, but how do you really know? In the 1980s gay marriage was universally seen as an absurdity as well.
Old people get married, so yes, the ability to reproduce is completely irrelevant.
