Fortunately, in most cases of infidelity, the cheater is with someone the partner doesn’t know. In these cases, I’m always saying that the focus should be on the fact that the cheater is the one who cheated, and that is the person the blame should be placed upon. However, there are cases in which it makes perfect sense to place equal blame upon the other woman or man. Those are the cases of ultimate betrayal.
I was watching Shania Twain talking on a show about her husband cheating on her. She was, of course, hurt because her husband had betrayed her, but the focus of her anger seemed to be on the other woman (surprise!). But in her case, it made sense. Why? Because the other woman was her friend–the woman in whom she was confiding about her problems with her husband! 

In a case where the other woman is a friend or family member of the betrayed, not only is she betrayed by the cheater, but also by the friend or family member who is supposed to care about her! In a case such as that, both the cheater and the other woman are equally to blame, because they both have a relationship with the betrayed, and should both consider her feelings enough to avoid getting into a situation where an intimate relationship could develop.
You can say that a total stranger should also consider the feelings of the partner of the man she loves (or with whom she is sexually/romantically involved), and that’s true. But how much more should she consider the feelings of a friend or family member?
In Shania’s case, the husband of her best friend (her husband’s other woman) told her about the affair. She didn’t believe it at first, but it was confirmed, so she was trying to not only learn to deal with the fact that her husband wanted out of their marriage because of another woman, but she was also trying to deal with the fact that her best friend also betrayed her.
At the same time, however, she did admit that the marriage had suffered greatly because of her career, and that the intimacy she had once felt with her husband was gone. She said they no longer communicated as before. From the way she described it, I would say the marriage was basically over when the affair began. I’m not saying that excuses anyone’s behavior, but it does shed some light on how these things happen. That doesn’t make it okay, but it does make the vulnerability much more understandable.
Especially in a case such as the one involving Shania and Mutt, you have to wonder, What the heck was he thinking? She’s Shania! She’s gorgeous, she’s talented, she’s intelligent, she’s… Shania! But she wasn’t there. She was on the road. She was busy, and their relationship had changed drastically. In the beginning, she was a young woman who was dependent upon him. He practically built her career with his own hands. And that’s not to take anything away from Shania. She’s very talented, but he’s Mutt Lange! She went from a new-comer to Nashville with a first album that went nowhere, to doing things that hadn’t been done before, and being on top, which was largely due to Mutt’s co-writing and producing her The Woman In Me CD (which I love).
Shania then became a star and no longer needed her husband in the way she had before her career took off. I’m not saying that’s the reason for the affair, but I’m saying that when the dynamics of a relationship change in such dramatic ways, there’s always a chance that one or both of the parties involved will not feel the same toward the other. Mutt went from being the star in the relationship, to being “Shania’s husband.” I mean, he’s never been the type to crave the spotlight. In fact, he shies away from it. The fact that she was thrust into the spotlight, becoming such a sensation, also could’ve contributed to the demise of the marriage.
There are so many dynamics that make up a relationship, not to mention our personalities and everything that makes us who we are. Sometimes the things that once felt good to us are no longer desirable, or even comfortable, to us. And sometimes the things you need are no longer found in your partner, but you do find them in someone else. These are all issues that make us human, and thus, imperfect.
Every case of infidelity is humiliating and difficult, not to mention painful. But in the cases in which the other woman or man is someone close to the betrayed, the pain is multiplied many times over. This ultimate betrayal makes the prospect of trusting anyone much more difficult than it would be for anyone who had been betrayed by a partner who was with a stranger.
If you’ve been involved in this type of relationship, I strongly urge you to get some therapy. I believe that therapy is beneficial to everyone, but in a case like this, it is really necessary in order to learn to cope. Please get help for yourself, and for the sake of your children, and for your future relationships.