You are claiming that this is the primary reason why we have state-regulated marriage? Two words: [citation needed].
UPDATE
@downvoter, if equality isn't a fuzzy premise, where is the line drawn between things that should and should not be equal?
For instance, why shouldn't everyone have an equal opportunity to participate in the Olympics? Why shouldn't everyone have an equal opportunity to be a CEO on Wall Street?
It's because they do not have the functionality that these positions require, as mynameishere pointed out regarding marriage. Claiming equality merely glosses over these important distinctions and does not bring any substance to the argument.
Sure, maybe same sex relationships can be artificially generative, but this is no where near the capability or likelihood to equal the basis for traditional marriage.
So, same sex relationships should not have the same benefits that are accrued by a generative relationship, since they do not require them and are thus just squandering resources.
Emotional issues are important, but they should not be the bottom line in a situation that clearly has a functional basis. However, the pro side seems to be mainly emotive in their driving arguments.
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.