Interesting also that you assume I have no experience with the situation. Is this because I assert that an outsize reaction isn't warranted? Yes, of course it's hard on all concerned, and usually regrettable. The thing that I don't believe is that an outsider ever knows what goes on between two people. Even the people involved usually don't understand it. So there are two problems with making someone else's marital behavior one's own problem. Firstly, an outsider is rarely in a position to judge. More importantly, even if one could judge confidently, taking on other people's marital problems, or other failings, as central issues in one's own life, is a huge red flag. Infidelity is just a special case--it doesn't make sense to take on the misbehavior of others as a central fact in one's own life regardless of what the behavior was.