Perhaps it is only the reason Justin said to Harry: they wanted a full trained wizard in the family, even they pretend not to say so. I mean, they can't ignore the wizard world, they know how powerfull and dangerous is. Just pretending not to mention anything about it, they can't be freed out of it. So, as a mecanism of defence, they want to ignore, but inside of them, all they wanted is to feel safe, and Harry can help them because he's the boy who lived.