Incest is one those things, like pedophilia and beastiality, that not only potentially indicate a harmful mental illness but also can directly harm someone.
In the case of incest it can very easily lead to malformed children being born. There's also the case of people performing incest on mentally challenged members of their family. Incest can completely scar a person for life even if they consented to it upon retrospect.
No it wouldn't.
During the Victorian era and before that people had the thought that your worth as a person was determined by who you were born from. This is largely why in-breeding was a thing because people thought poorer people were inferior by their very nature. It's why royalty has always been determined through birth and not ability.
Nowadays we have science and genetics and can prove if you reproduce with someone in your family it leads to less genetic diversity. And this is very commonly known. If Incest was legalized today nobody would intentionally do it to avoid soiling their line with "bad blood".
