Several ancient societies allowed, or even encouraged, marriage between siblings or fathers and daughters. How was this even possible, when humans seem to have such an innate revulsion at the thought of close-kin incest?

I’m specifically thinking of ancient Egypt and Persia, which have many examples of close-kin marriage, at least among royal families. Indeed, I’ve even seen sources that flatly state that Zoroastrianism (the state religion of Persia) encouraged marriage between siblings.

But, i always thought that it was universal that humans feel innate disgust at the thought of sex with such close relatives. Is this true? If so, then how, and why was this allowed?

Not to discourage any new answers (especially for Persia) but you might like reading /u/cleopatra_philopater's answers on similar topics:

In societies where incestual marriage with very close degrees of relation was tolerated/celebrated (eg; brother-sister pairings in Egypt) did people ever notice or point out that the children of such pairings seemed to die more often than ones produced from non-incestual marriages? and a follow up on close kin marriage in the ancient Mediterranean (Greece/Rome)

But these mostly talk about the effects of inbreeding and when/where this was acceptable, there isn't as much on why or how these practices first emerged. Also it talks about close kin marriage in Roman Egypt, Greece and Rome not Persia.