[cyorir]

The main argument against incest that I know of pertains to recessive genetic diseases. Suppose one parent has a recessive disease, and the other does not (P generation). Then F1 generation may have carriers, but will not display the disease. If F1 mate together, however, then the F2 generation may contain individuals who are homozygous for the recessive allele, so they will display the disease.

There was a case in which this happened, the "Blue Fugates" [0], who had a particular recessive disease[1].

[0] https://en.wikipedia.org/wiki/Blue_Fugates [1] https://en.wikipedia.org/wiki/Methemoglobinemia

This argument made sense before genetic screening was available, but falls apart with genetic screening, since you could screen the siblings to see if they are both carriers.

[DanBC]

The other argument against incest is to protect vulnerable people from abusive adults.

There are enough cases to realise this is something of a problem, although protections can probably be built into the system.

[JoeAltmaier]

And other forms of marriage are not subject to abuse? Doesn't rise to the level of prohibition, I think.