Thinking something is gross doesn't make it immoral.
Your logic about why we should ban incest could perfectly be applied to gay sex. In fact theres a study that shows that men's brains react to two men kissing as if they were watching worms.
Also saying its instinctive is arguable. Id say its more cultural than anything. In fact among some islander tribes it's accepted.
The main reason why its frowned upon is because of the possibility of malformed babies. And this is out of the equation in this case.
But the reason behind the taboo has been forgotten and only the automatic rejection remains.