[hits crack pipe]
Strictly speaking the consequences of decriminalizing incest wouldn't have any significant ramifications on society and it's mostly illegal just because people think it's gross.
>Muh genetic defects
This fear is grossly over exaggerated, while incest does indeed carry a greater risk of producing children with birth defects it's still uncommon within a single generation of inbreeding, the issue arises from multi-generational instances which outside of highly isolated groups or intentional breeding programs are unlikely to happen. The vast majority of people are kinda psychologically hardwired to find sexual relations with close family members highly unappealing so incestuous relations would naturally be way too uncommon to end up with situations of a family being straight up inbred for several generations unless they're specifically going out of their way to make sure it's "kept in the family" or live in some absolute bumfuck village where it's nearly impossible to talk to anyone who isn't related to you.
[takes another hit and starts visibly shaking]
>Muh grooming
This is a risk of literally any relationship where one party hold significant societal or physical power over the other, a teacher, a coach, an older classmate, a friend of the family; and even then a groomer within the family can still easily groom someone if it's illegal and just groom them into keeping quiet about it, if anything legalizing incest would make highly questionable relationships easier to spot and scrutinize, which would allow them to be easier to intervene in if it was believed the relationship wasn't as consensual as the abused party was convinced it was.
[takes a third hit, shakes become notable worse, eyes becoming severely bloodshot]
In short, you should just let me fuck my precious Snuuy, he belongs to me.