A lot of couples lose attraction towards each other because they let themselves go. Attractive & lean people tend to take their health for granted in their youth and the bad habits catch up fast. 60% of america is fat as fukk and that shyt isn't attractive. Also as you get older, uglier and more unhealthy, your self-esteem drops and you might not feel comfortable being objectified because you no longer see yourself as sexy. This is not true for the ones who take care of themselves but they are the exception. Most of america is fat and gross.
Most men in America aren't ambitious and wives get turned off by having to put on the pants in the relationship. Normal guys wake up, go to their job they hate, come home and drink a few beers, watch TV, then go to bed. Weekends are spent cleaning, doing groceries, taking kids they hate to sports, etc. Wives start hating their monotonous life - a fling with coworker/etc brings back some excitement into the mix for her.
Everything else in this thread is true but a lot of the blame falls on the loser ass men, not just the woman.