This is basic sexual psychology, and not at all uncommon.
Remember, a single woman is looking to find the best possible father for her children. Just as mainstream pornography focuses on the male ideal of unlimited sexual access to young and nubile women, mainstream romance novels (which are basically porn for women) focus on exclusive sexual committment from a strong and powerful man. I've discussed elsewhere the underlying drives that lead to this divergence of interests of, but what it boils down to is - a man will happily fuck a woman he wouldn't consider marrying, while a woman will reluctantly marry a man she'd rather not fuck. Research has shown that about 1 in 10 men rate "alpha" in the eyes of women, which mean that 9 out of 10 women are compromising their instincts in their mate choice.
Which explains all those wedding pictures where you can't understand how such gorgeous young brides wind up beside these unpromising dweebs.
So you might think such women would be eager to upgrade their genetic chances by stepping out on hubby. However - marriage is a huge deal to a woman. Her instincts and all of society tell her she should be marrying Prince Alpha von-Studlyrich. Winding up with Larry from Accounting represents a huge failure on this level. She ranks a lot of her self-worth based on her mate choice (so do men of course) - but unlike a man who has a one-nighter with a coyote-girl and doesn't tell his friends, her choice is both public and permanent. So as is common in humans, denial kicks in.
"He's got a great future, he's good in bed, he's well hung." she tells her girlfriends and herself. Actually all of these things are woefully average. Nevertheless, she will resolutely defend her man, and strongly resist the idea that she could do better. Because - if she do better, she SHOULD have done better, and she doesn't want to believe that.
So - wives often take a lot of convincing to overcome this, and it takes them a while to adjust to the idea that they really can have the best of both worlds - support from Larry Loser, and sex from von Studlyrich. While they're going through the transition - they're going to have to deal with the cognitive dissonance that happens when their actions don't line up with their internal belief system. Hence angry, apologetic, and generally messed up.
Once they get their heads around it though - well, too bad for Larry...