Felony by definition means it's punishable by at least one year in prison.
So specifically, why are we incarcerating consenting adults for having sex?
From a moral point, don't do that. From a legal point, stay out of the bedroom.
I'll also add context of this is a very Western belief. Natives of America prevented inbreeding by not marrying within the clan. Your first cousins could be in a different clan and therefore open for marriage.
It largely depends on how prevalent it is socially.
In societies where first cousin reproduction isn't common, the increased risk of birth defects is about the same as a pregnancy where the mother is in her 40s.
In societies where it is common, the rates can go up sharply though when it compounds across generations.
I think the issue is twofold: if you allow a groomer (a real one, not a drag queen) to be around a child for their entire life, there will be an effect on what the child wants as an adult; and people have a lot of opportunities to blackmail family members.
Those things make it difficult to determine if the two consenting adults are both truly consenting. Itās the same logic behind banning polygamy- itās very easy for people to become trapped in it because their entire social universe supports it, so if you say no, youāre excommunicated. Thereās therefore no real way to know if you consented.
Itās the same reason cops canāt have sex with people they are holding under arrest. Oh waitā¦
Having kids with your 1st cousin is rarely a problem unless it's a pattern that's repeated over several generations. I still think it's kinda weird, but as you say, that's a cultural aversion, not one that rises organically out of fear of inbreeding. That said, the incest taboo is one of a handful of human universals, like music, language and fire, to name a few others.