Why is incest (sibling) looked down upon? what if we could fix its downsides?

In my story I wrote down how they had a way to fix the genetic consequences of inbreeding.
But to make a realistic response by the civilians I want to know why it looked down upon and what if it’s consequences weren’t a problem?
Would it still be hated.
Only siblings, not any other variation of incest
Why is incest (sibling) looked down upon? what if we could fix its downsides?
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