Statesboro, GA, USA
Are there many examples in literature of this? It is interesting. I can only think of one other, though the relationship is essentially selfish and negative ... Fyodor Karamazov and the idiot girl whom he has sexual relations with, in "The Brothers Karamazov". The book you linked seems more of a positive example.
I certainly see nothing ethically wrong with an intellectual mismatch between lovers, for there are many worthy qualities other than intellect. I suppose if it is genuine love, and not some veiled attempt to take advantage of a vulnerable person, one would have to be prepared to accept and face the social stigma about non-conventional romances that would come about ... the same goes for lovers whose ages differ a great deal. In todays culture, a man who truly loves a younger woman would be called a sex-seeker, and the woman a gold-digger. These judgments are unfair simply because they don't take the possibility of love into account, and they are uncharitable, tending to think the worst of others.