Sacred Haven of Debauchery

Inbreeding explains the ban on sexual intercourse itself, rather than on bearing offspring, because the use of contraception does not exempt one from criminal responsibility (hence the inconsistency). As for the social aspect, in countries where there is no ban, there is no observed increase in **incest between relatives, so this argument also doesn't hold water.**

I believe that, in many ways, this is due to the fact that they already have reduced chances of having offspring...

But not zero chances. In the end, it all boils down to society's dislike of this phenomenon rather than rational reasons.