Incest avoidance actually a deeply rooted biological mechanism that precedes humans existing as a species. Even organisms that lack a central nervous system have mechanisms to avoid this type of sexual fertilization. The only "socially constructed" bit is that you find this kind of content arousing because of the taboo.
Not very familiar with history are you?
Mankind has been reproducing through incest for quite a long time.
Until large nations rose up, it was mostly small tribes. Aside from the occasional eradication and capture of women, how do you think they reproduced to increase their clan/village size?
Royalty lines have been reproducing incestuously since the middle age at least (too few records before that to be truly accurate), and the middle age bit was pretty well documented and known. If Royalty did it, do you really think the very poor farmer population, isolated with no money and no future would be THAT picky about who could bear a healthy son to continue the bloodline?
Virtually all Roman and Greek gods, were incestuous, which tends to suggest this was ok at the time (if the gods do it, why shouldn't we?). The bible also has its fair share of incest in it. Even in ancient Egypt, it was pretty common too (this has actually been documented well enough.)
If anything, incest has only become problematic "recently" (taken in the against the time frame since mankind began.)
AS for other liforms having mechanisms against it, it sure doesn't seem to apply to cats and dogs... The alpha male certainly doesn't seem to have any issues mounting his daughters and granddaughters. Not to mention Lion pride with their "one penis limit" in a pride... I'm not seeing your biological barrier in live examples so far
Another thing I just remembered, even from ancient time, it's marrying that is not allowed. Until quite recently, sex was not a legal issue between relatives, only getting married (or getting the kid recognized legally.) Remarkably few countries have actual criminal laws against having sex with a relative.
Here's a map from 2018 of Europe's legality on the matter, to the best of my knowledge, most however, have laws against getting married or getting the names on kid's birth certificates: