I remember having read this same thing myself, and it makes some sense. If memory serves, several studies indicated that nature wires us not to be incestuous, but interestingly enough, also not to be too "out-cestuous." Thus the yuck factor sets in at the thought of sexual relations with people very closely related, but also with other species, or indeed to some extent with people very remotely related.
As to the 'yuck' part of your brain, there have been studies that show children raised together (who are unrelated), are unlikely to develop romantic relationships with each other. One study claimed that being raised together changes brain wiring (that's the technical term) to prevent incest-related birth defects and such and such. Of course, the study couldn't possibly account for societal pressure to not engage in such relationships, but whatev.
I'll see if I can track down one of the studies, if anyone is interested.