The actual mechanism of revulsion is almost certainly an evolutionary trait.
Any trait that would tend to stop someone from going down a reproductive dead end (same sex, post childbearing/child rearing age) or undesirable (closely related, obvious inheritable disability) would be advantageous to the passing on of the genes of that person.
Since once you are sexually excited about someone/thing it can access the very powerful sex drive it would clearly be beneficial if there was anything in a persons makeup that would inhibit them from pursuing one of the above mentioned dead ends.
Someone with a revulsion powerful enough to stop them succumbing to the sexual attraction of another with whom reproduction is impossible/undesirable would certainly be in a better position to pass on their genes.