What a strange analogy. Evolution is a natural phenomena, while marriage is a state-regulated union. There is a set of reasons apart from tradition why legal marriage exists, and very little in that set would apply with any frequency to "gay marriage". The most obvious reason, of course, is that the division of labor between the sexes is inevitably derivative of the constraints created by pregnancy.
Brin doesn't really argue beyond this,
we see this fundamentally as an issue of equality.
But this is an error. It's an easy error to make.