The definition of normal does indeed change. By your logic, slavery was always a good thing because at one point, it was normal.
Perhaps I should say it's standard thought to think incest is wrong? Chances are not 100% that an inbred child will have issues. Wanna try it and gamble? They might have a perfectly healthy child, or it could need to live in the hospital/be mentally ill/etc.
It's morally wrong (this is something most people -will- agree with) to like a family member that way.