It is wrong because if it is that bad, then she should be seeking out professional help to help her untangle herself from a potentially abusive relationshiop. If the husband is emotionally abusive or physically abusive, then either she get counselling to help her figure out how to be strong enough to leave, or if she thinks there is still hope, to get him to get help. Having an affair with another guy doesn't change the abuse or make her safer.
If her husband is cheating on her, or they have no sex life, or he is a selfish ass that is wham bam thank you ma'am, and it is important to her, why stay married? Or why put up with it? It is one thing to understand why someone cheats, why someone goes outside the marriage and it isn't always simple, like a guy with kids who stays in the relationship with a wife who is not warm or anything, who realizes that if they get divorced it would cause havoc he can't face, and he cheats to try to survive, is a lot more understandable than a horndog who thinks he is god's gift to women (and btw, same thing with a wife with a shit husband). The problem is when you go outside, it means the primary relationship has little to hold you there other than comfort or economic necessity or whatever, and what do you really have?
It isn't black or white , just saying going outside, getting into a relationship with someone else, is not the same thing as having a loving married relationship, going outside doesn't solve anything. Yes, some of the person's needs are met, but if the person you are with is that good, then get divorced and marry the other person.
What makes this tricky is something I read in a pretty good book about couples dynamics, that with many issues of intimacy in a relationship it is a two way street, that usually it is both people have something that is hindering it (and when I say intimacy, I don't mean just sex). He gave a great example, where a wife was upset that her husband never stood up to his family, defended her, said he was weak. The therapist pointed out that she wanted him to toughen up and stand up to his family, but what she was doing was complaining about him not doing it, instead of helping to build him up, help him get strong, so he can. In the case involved he said the wife thought she had no responsibility, when in a couple it has to be from both. Or the wife married to a man who she complains is emotionally distant sexually, that he is off in his own world, not interacting with her, but the therapist points out that by her own admission, the wife has trouble with being touched in sex or expressing her pleasure. The author claims rarely is it one of the people who are having trouble. In the context of this discussion, going outside doesn't solve the fundamental problem, and that likely the outside relationship is no more intimate than the marital one, that the wife sees it as such because the sex for whatever reasons is good, but the reality is it is just sex even if she thinks it is more.
