Yes. It’s actually is a phenomenon for middle aged men AND women. The reality is that they are actively unhappy they’ve just been conditioned to believe that it’s okay, a normal part of being married so they are actually happy. Or they are confusing security/safety with happiness, you can tell the difference by assessing if that security is accompanied by a constant irritation in your soul. If it is then you’re not happy, you’re just safe. The other, and I believe more relevant scenario, is that conventional marriage doesn’t account for the very real, and scientifically proven fact, that human beings can and do love multiple people, at varying degrees, simultaneously. Check out the work of Dr. Helen Fisher. Many couples are beginning to acknowledge this fact by engaging in consensual non-monogamy (both sexually and emotionally) within the context of their relationships. For some this is a by product of seeking to answer some very normal and vital questions brought on by mid-life. Once those questions are answered they evolve either inside of their existing relationship or they move forward in a completely different direction. For others it’s the adoption of an entirely new lifestyle of non-monogamy. Whatever it is, I believe we need to make space for this part of our nature and life cycle. Much damage is being done in the hidden recess you outline here.