Obesity epidemic. I mean, when so many people are that unattractive, it's not exactly rocket science why we're having less sex.

EDIT -

The number of replies by people in denial that being horribly out of shape and having excessive body fat isn't related to attractiveness and sexual enjoyment is... well not surprising. Though it is indicative of how in denial our infantilizing culture is and how effectively it enables people to live out slothful, narcissistic, and self-destructive lifestyles, while blaming others for not accepting them.