Apparently I misread his post, but (and this coming from a queer who is not particularly interested in incestuous relations), the comparison to homosexuality stands. Biology has put a block on Incestuous behavior in a similar way to how it puts a block on homosexual behavior. The majority are not inclined to it in either case. In some fraction of the population, for whatever reason, sexual development is different and they become inclined towards sexual behavior that isn't appealing to the majority. In both cases you can read the fact of their difference as evidence of pathology, but in both cases you'd simply be using an impoverished naturalistic morality in order to put a fig leaf over your unjustified taboos.
Incest may be more profitably compared to bisexuality, in that, so far as I am aware, the socially discouraged attraction is not exclusive. And we do not limit our activism for queer rights to only exclusive homosexuals, and for good reason.
I think you're misunderstanding the "Westermarck Effect" theory. It's not really a biological block. It's a psychological block that's developed during early childhood. If you were separated from your sibling at birth and reunited later, that block would have never formed and such people were documented to have struggled to reconcile their sudden sexual attraction. And on the opposite end of the spectrum, two kids who are not blood-related but are raised in the same foster home (ie. one adopted) from ages 1 through 6, have shown the same psychological blocks and aversion to any sexual relations toward each other that normal siblings would have.
It's only biological in the sense that all (or most) humans develop this psychological block during early childhood development, possibly as evolution's way of preventing inbreeding and diversifying the gene pool. Unless you're saying homosexuality or sexual orientation is something that's developed from an early age, rather than being born with, it's not similar at all.
I think you're looking at this as is if the "psychological block" was placed on us solely because of society's mores today. But studies of past human societies and civilizations show that nearly all human societies in history, even when isolated and independent, still placed a strong taboo against incest within the nuclear family, especially parent-child.
Of course, this could all just be a load of bull as I'm not even sure if the Westermarck Effect is accepted fully by psychologists.
I think the GAF defense force that's surrounding this though are looking at it from the wrong angle -- like "who am I to judge how two consenting adults act?" or "I wonder how I would act if my mother looked like Sofia Vergara or my sister like Lacey Chabert." They're looking at it from outside-in, without that psychological block that makes incest seem so disgusting and abhorrent when viewing our own parents and siblings. Then again, who knows? Maybe if your mother was Lisa Ann or your sister was Kate Upton, that "psychological block" starts crumbling down at the onset of puberty.