How many couples do you witness in therapy? I feel like youre just making this part up, maybe based on a handful of personal experiences with your own people and shit.
This is natural. Many couples who end up in therapy say something like "we became like brother and sister". It's the body's natural reaction to being intimate with one person for a long period. Your genes are telling you "enough! We want sex with someone new!" and the chemistry that you had with your partner when you just met disappears.
sounds again like personal experience with nothing even resembling a real study of a population.
Why do you think so many married couples have sex once a month or less? Again, more proof that monogamy is unnatural. If we were monogamous we'd keep humping for the duration of the relationship as we do in the first week.
The couples that do fit this bill are probably super-rotted physiologically and just straight unhealthy and awful. THEY are diseased, not necessarily monogamy.
"Ah, those endless forests, and their horror-haunted gloom! For what eternities have I wandered through them, a timid, hunted creature, starting at the least sound, frightened of my own shadow, keyed-up, ever alert and vigilant, ready on the instant to dash away in mad flight for my life. For I was the prey of all manner of fierce life that dwelt in the forest, and it was in ecstasies of fear that I fled before the hunting monsters."
Jack london, "Before Adam"