Like most things, this one is complicated.
Genetically parent child relationships have a very large chance of a birth defect. Last I looked on the order of 30%, and it clusters around the genetic intermix of the father and mother. So parent/child pairings who have damaged kids will usually have damaged kids if they have 7 or them, and a normal kid indicates the pairing will usually have normal kids.
Brother/sister pairings have much fewer issues, and cousins are almost immeasurably more likely to have issues. Second cousins have no birth defect increase.
This situation gets worse as more generations partake.
It exists as part of human nature because having an offspring with a family member is immensely less desirable than having one outside the group, but biologically preferable to having no offspring. For example nomadic tribes and isolated populations.
We think it’s “weird” for the same reason most consider homosexuality and underage sex highly inappropriate. One does not serve the reproductive purpose, and the latter usually resulted in the death of the mother if conception occurred.
God I’m glad I don’t do psychology anymore...
As such we disdain inbreeding because of its damaging aspects to the children - something I find quite odd in a society where abortion is perfectly legal.
I can imagine this being a line in a Doctors office in 20 years
“Oh god, Timmy. I’m so sorry. I didn’t mean to accidentally brush up against your balls. That’s immoral. I’m just looking for a vein for the Lethal Injection I have here. Your parents both lost their jobs so they sold us your organs for rich people to buy...thank god for the new ‘putting unwanted children to use’ laws...”