What also shouldn't go unnoticed, while in incest discussions the most used argument is the genetic one - which, like said by multiple posters, is mostly a problem if it goes on for generations or randomly happens between people that just happen to contribute some already messed up/ill/'contaminated' gene pools, the much more important direction is the overall gene pool limitation it can lead to.
Going with the 'the bible already contains incest' argument, the people involved there should have had almost perfect gene pools at that time(s), and so them having incestuous relationships seems preferable to the extinction of mankind until there is enough diversity in the gene pool to end those circumstances.
It's like people married off their daughters two villages/valleys further and kept track of relationships to keep the genes spread out and healthy (or offered their daughters/wives to strangers to get the foreign genes into their pool) but before your village dies out, marrying off your daughter to your neighbor might be an acceptable idea.
Then there is the point of abuse that botc76 mentioned that I feel while not discussed that often is actually the much more relevant one in our day-to-day life.
In most cases, it happens between family members with a power imbalance between them/dependency of one of them to the other - which can lead to abuse of that power.
So this is not as much about keeping mankind healthy as it is about keeping our children safe and unharmed. And that point is not related to blood, but the family environment.
Besides catering to the American mainstream, that to me seems comparably prude, at all like in 'it's bad to have your company associated with something (possibly illegally) lewd', that would most likely be the reason for states or institutions like Patreon (because of PayPal (because of the credit card companies (because of their board members (because of their (companies) standing)))) to also include step relations, adoptions, guardianships and whatever you can think of if it means you live together, as incest. Just makes it easier to cover all this under one umbrella.
