A string of emails suggesting an affair are sent to the boss of one of the parties. In law, medicine or banking, the recipient would probably quietly have put the emails in a file, pacified the husband, and got on with doing his job. But, as we saw last week in the case of General Petraeus, the military is different. So, instead of sighing "private matter, none of my business", the boss circulated the emails to fellow senior officers and other colleagues, causing widespread outrage. The man in question was denied an expected military honour and shunned by the arbiters of regimental morality. His marriage was over, but instead of being offered support, privacy and security by his colleagues, he is an outcast.

How many judges, QCs, bankers, surgeons are still with their first spouse? Good manners, or political correctness, make it bad form to pretend an understanding of someone else's marriage. Not in the military, it seems. And if you do take a stern line on personal morality – a perfectly respectable position – be sure your own house is in order.

If you were to attend, say, a Remembrance Day service among military families you would assume that these formally dressed, shoulder-to-shoulder couples were untouched by human weakness, almost from another generation, with their virtues of hard work, fidelity, sobriety and godliness written on their hearts. This is far from the truth. Infidelity happens on both sides. Divorce is not inevitable: with the physical separation that army life brings, why bother?

So there is a low-key hypocrisy, in that men and women are unfaithful to their spouses, but what you mustn't do, paradoxically, is "do the decent thing", be honest and get divorced. Army marriages are demanding for both partners. The men are physically, mentally and often emotionally exhausted, the women have learnt to be self-reliant and are often resentful at being left alone with the children for months on end. But when these marriages work, it is impressive and heartwarming.

Too often, though, high-achieving men can get puffed up and removed from the humanising influence of the banality of everyday life – small children to humour and clear up after and a wife to make them laugh at themselves and tell them they are being boring. Instead, they are served by subordinates, their every domestic need met. The resulting unchecked narcissism allows them to start believing their own narrative; either that a little extramarital nookey is forgivable, or: "You cannot leave your marriage. It is wrong. Because I am in charge..." It reminds me of Eddie Izzard's "Death Star Canteen" sketch: "I will have the penne arrabiata". "You'll need a tray". "Do you know who I am? I am Vader, Darth Vader, Lord Vader. I can kill you with a single thought!" "You'll still need a tray". It is a brilliant observation on self-importance and a lost touch with reality.

Top generals can become lonely, slightly tragic, figures whose judgement goes unquestioned. Their moral code is gospel, yet they fail to recognise when they themselves have broken it. We should not be surprised by the mistakes made by General Petraeus; alone for hours with an adoring biographer who shared his interests and listened to him (possibly) bore on about himself for hours. We may wish him to be Superman, he may even have believed it himself. Yet there was no canteen server to remind him that "the food is hot. You'll need a tray to put the food on". And no one to offer him forgiveness for being a mortal underneath his uniform.

Jane Digby (a pen name) lives with a senior former soldier