It's taboo because the child produced by incest has genetic flaws...reduced IQ to the point of mental retardation, teeth size and extra teeth growing in weird locations, webbed toes and fingers, and really bad mental issues to the point of be a psychopath, and etc.
Sometimes the flaws skip a generation but they show up .
I like playing games like these but please do not try to normalize incest.
"Normalize Incest" hmmm... so let's take a little stroll through bible mythology shall we? Since Adam & Eve were supposed to be the first of mankind the only way that more humans came into being was through incest of some sort because there were no other people specifically created by God in this story, it follows that all of humanity was created through the children in Adam & Eve's family.
Cain & Abel had wives (where'd they come from if only Adam & Eve were created? They could only be their own sisters, there's no other explanation for their existence). Continuing on that theme when the great flood killed off most of the world's population and creatures supposedly Noah's family was all that was left of mankind & if so all of humanity sprang from said family which again would by necessity of zero alternatives include children of some degree of incest (cousins, nieces, nephews, brothers, sisters, and so forth).
Granted this goes off of religious storytelling which is deliberately vague in offering an explanation which doesn't involve incest for how humans procreated & grew in population.
If you look at sexual practices in nature incest is absolutely a commonplace occurrence when there is a lack of other suitable breeding mates outside of one's immediate family group.
Science has accurately shown that amongst mammals the breeding partner which is usually the most attractive to the individual in search of a mate are the ones with genetic traits that most similarly mirror those that the searchee also possesses.
This is the reason why in human relationships it's absolutely normal that men are usually attracted to women that strongly genetically resemble their mothers or sisters, and for women those that resemble their fathers or brothers. I'm not referring to physical appearances, but genetic similarities which for some reason many species are able to somehow intuitively perceive.
From a genetic point of view being attracted to one's biological family makes sense because sharing these genetic similarities strengthens them (the good and the bad).
The problems arise when there's too much concentration of the same genetic material as it leads to the likelihood of mutations but that generally only happens through successive generations of inbreeding without any outside genetic influx.
This isn't "normalizing incest" this is acknowledging that in fact in nature incest is anything but abnormal or unnatural even if it causes distaste & judgement in human society, that doesn't negate that it is a phenomenon that occurs frequently in nature itself.