The number 1 cause is weight gain. I am 62, my wife is 47 but looks much younger. We have been very happily married for 18 years. Our schedules are very different, but we still have sex twice a week. That is very good for my age.
However, after she had my son she gained a lot of weight and there was a period where my attraction to her was greatly diminished. Therefore I would say weight gain is the number 1 reason for a loss in intimacy. My wife got back into eating very clean/healthy and she works out consistently. Our son is an elite athlete and I am a competitive body builder (last competed in 2018). So the whole family is into fitness. She is actually considering Ozempic to lose the last 10 lbs.
The second I would say is age, I am not attracted to many 62 year olds. My wife still get carded. Quite frankly I consider myself ugly (turning 60 beat y face with an ugly stick, LOL) but my wife is still quite attractive.
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So guys, always marry younger and go for a woman that eats right and exercises.