Well, it’s not. In my experience the problem is that it’s not a two way street. Women don’t suddenly go blind to the opposite sex when they get married either.
Yet find me a married woman who has not been harassed or pecked at by her husband because she smiled hello at or spoke to a man, a man noticed her, she turned heads, etc. It’s going to be an pretty intense search, best pack a lunch.
They’ll start to pick at their wives over their attire, push food at their wives so they’ll get fat (there was actually a study on this years ago), or try to sabotage her efforts to look good either financially (you’re spending too much money on make up), emotionally (you should only worry about looking good for me) or religiously (Godly men only desire modest women. Married women should be modest).
Anyway, I betcha that subconsciously it’s a whole lot less about insecurity as it is about 'if you’re going to ride my ass about being a beautiful woman day an night then I’m going to ride yours about paying attention to other beautiful women’.
That study I mentioned was aired on a segment of one of those news magazine shows like 60 Minutes. It found that over half of men started pushing their wives to overeat after they got married and the more attractive she was (the more notice she got from other men) the more likely it was he would do this.
My ex husband did this. Most of our fights early on were about him pushing food at me, and me saying “thanks, I’m full; it’ll keep for lunch tomorrow; I don’t want anymore" and then escalation because of trying to control and not listening.
I saw that program with my Grandma after I got divorced, which was why it resonated so well. Until then, I just thought he was being an ass. I mean, he was being an ass, but I didn’t realize there might have been something behind it due to his own insecurities.
Anyway, it’s recognized as a form of human mate guarding. You guard your mate by making her unattractive to other potential mates. Sick and underhanded, isn’t it?
Truth is, once you get married none of us are ever 100% secure from losing your mate to another. At least none of us have to stay with an unfaithful spouse anymore. Still you’re right about it being a loss of investment for either party. And guarding behavior based on insecurity tends to trigger the same behavior in the other spouse.