Why is even consensual incest taboo?

You need to go back a long way, probably the New Stone Age, to find the origins in tribal laws. Exogamy - marriage outside the tribe - was often the rule. But there were certain tribes who insisted on marrying within their group. It does seem likely it had been learned that a people slowly weaken with inter-marriage, but there were other reasons too, based on religion. It's very complex. The disgust at such things show how deep the feeling is after so many generations. The risks of deformities are very low and far too much is made of this. The problems of continual intermarriage are much slower to show up.