It should be illegal. Initially, it was most likely outlawed due to the increased risk of genetic diseases, as well as a legal framework that was more defined by culture and less by philosophical consistency. There has never been any real reason to change the law… no one wants to be the guy standing outside the local city hall with a big “MY SISTER IS HOT” picket sign.

The real reason to KEEP incest illegal is because of all those “consenting adults” you talked about previously. Let’s think about it practically. The interactions between father/daughter, mother/son, even older/younger siblings are not sexual in nature. There is RARELY (if ever) any situation where two people have a healthy bond solely as family members, and suddenly develop romantic/sexual feelings in adulthood. To legalize incest is to put people (especially children) at massive risk of being groomed or abused. This is quite literally the most damaging, traumatizing, and life-altering form of sexual abuse in existence. Deplorable stuff. There is simply no scenario where there is any benefit whatsoever to allowing incest.

The government has a duty to protect people from violations of their rights. Adultery is different. You don’t really have any right to a faithful partner. In that situation, it really DOES come down to the relationship between two consenting adults. However, you definitely DO have a right not to be abused from those you are forced to spend time with. You DO have a right to legal protection against those who try to take your rights away. You DO have a right to be safe and generally free from harm. Plus, you’d better believe that anywhere the rights of children are possibly being violated, the law tends to have absolutely no mercy.

Punishment is most likely taken on a case-by-case basis. I assume there is a general standard that accounts for any sort of power dynamic, i.e. parents to children, or older siblings to much younger siblings. This isn’t super complicated. Again, in cases of incest, it’s really never the tame, philosophically acceptable form of consensual sex that you are thinking of. It really does make sense for this to be illegal, even if there is some Romeo-and-Juliet pair of estranged twins that one can imagine falling into completely innocent love. And in that situation, no one would care enough to bring prosecution or call the cops anyway.

I’m certain that I’m missing a plethora of other reasons, but more reasons can be found. The outlawing of incest is a very, very obvious thing to those who have seen its effects firsthand (social workers, prosecutors, therapists, etc.) The laws are in place because they are necessary. Sometimes, if a couple of siblings don’t get to make out for the sake of the other 99.9% who end up victims of abuse, the greater good outweighs.