Excuses are always there to be used, right?!
No matter what reason people have to come forward with mid-life crisis, it's a popular & socially induced idea that has its main basis in the people's fear to die. Now using it as an excuse to cheat ... that's just to easy. All a question of the kind of agreements couples have in their relationship. As far that I realized, when seeing friends of mine who have what is called an Open relationship, the definition of cheating becomes pretty different, they treat each other yet with an amazing degree of respect and love. Makes me sometimes wonder but I guess there's not only one way to define love in the universe than just the one dominant in our society. But for under the common concept, it's just an excuse ...lol... like someone saying that s/he was drunk and didn't know what s/he was doing, or something like.."There's a Gene for Cheating" ... that kind of crap. In both cases just like in many totally different situations of that kind, people know exactly what they're doing but eventually don't get the reasons "Why"!
Not being honest to themselves is one of them but ok, ... nobody's perfect!
But a Mid-life crisis in general has other reasons than cheating. I only presume that it's just convenient to use it as an excuse, for men just as for women.