I'm still impressed that this has remained to be one of the hardest social taboos today.

I don't see any problem with sexual relationships between blood-related adult people as long as very closely related ones (siblings, child-parent etc.) don't procreate due to the high risk of genetic dysfunctions and the heightened death rate.
There's no difference in the risk treshold between farther related family and people with heritable genetic disorders that are still allowed to procreate.

Psychologic or psychosomatic disorders in incestuous couples or children born from these connections seem to stem solely from the social pressure due to the taboo itself.

What justifies this stigmatization?