Use Occam's Razor.
I had not heard he had been paying her rent for years without his wife's knowledge. If nothing else, it has the appearance of an impropriety, doesn't it?
I can guarantee you that my wife would not be "thrilled" by it even one little bit.
I need a link on the part about paying rent for years without wife's knowledge. Do you have one?
Really, though, this is not the reason folks have moved away from Cain. It was the foreign policy gaffes, the state immigration gaffe, and the lack of constitutional knowledge that really hurt him. (I wasn't warm and cozy with his pro-choice verbiage - however much denied - even before that.)
The clincher for me was when he did the 3 hour debate with Gingrich and kept asking Newt for the answers. It made clear to me that even Herman Cain thought Newt was the expert in the room. Who was I to disagree with him about himself?
This race is over. Gingrich has risen to the top based on quality communication, debating, handling of the media, and policy substance. The remainder of the conservatives should drop out so Romney will realize that he's been smoked.