Uh, yes, I see it is VERY problematic that he did not tell his wife about giving money to another woman. Omission is the most powerful form of lie, to quote Orwell. Your opinion may vary as to the significance of Cain's admission, and it apparently does. But this is Cain's own admission, it is not inneundo like the past attacks. And if he can hide something like this from his own wife, what can he hide from the voters? It opens up a big ol' can of worms AFAIC.
So you are saying that he should have told his wife that he was giving charitable money to any woman, any man, any charity, or just this woman in particular? My wife expects me to handle certain aspects of our lives and does not want to hear about the day to day decisions I make, even regarding money. That's my job. She would probably get suspicious if I did start informing her of everything. She would figure I must be covering my tracks or something.
Just a question. Are you married?