There were implications of sexual impropriety; incest, for example. It was seen as being incompatible with the proper culture of our society.
So incest is cool and books, films, real life, and history. Just not video games. "Proper culture," eh? I see nothing proper about this culture. European royalty married cousin to cousin throughout history. Also genetically 2nd and more distant cousins have enough difference in genetic material that children born of such marriages are as normal as any other "Proper" marriage. Increase the fucking rating on this game if you have to. It is not like children play this and if they do maybe their parents should actually look into what their kids are doing instead of complaining after the fact.