@Ghekor
Out of morbid curiosity, why is it a no-go?
I mean I know exactly why certain aspects of it are extremely inadvisable, but I'm curious as to what your personal reasoning would be.
Obviously the core factor behind all the taboos against it is the increased risk of homozygosity across all alleles when inbreeding. But that necessarily means there is no actual problem with having sex with a close relative if, for instance, one is wearing protection, or they're the same gender.
And even otherwise, one needs to keep in mind the degree of relatedness. Selfcest isn't really an option, but that would be the worst for plants or other hermaphroditic species due to 100% genetic match. Parent / Offspring incest would be the second most risky with a guaranteed 50% genetic match. Sibling incest is in a similar situation, but the actual relatedness varies massively between 0% and 100%, simply averaging out at 50%. The further you go from there, the less relevant it becomes. First cousins for instance, being the offspring of a sibling of a parent, would have relatedness varying between 0% and 25%, which as we know a lot of cultures actually deemed a safe distance, though it would start to present a problem if done repeatedly.
Of course while I'm not personally bothered by the taboo of incest in theory, my family members all seem ugly to me, so I wouldn't anyway.