B.C.
I use to work with someone who summed it up like this:
When you're first married - you have whole house sex - anywhere anytime.
When kids come along, after they are a few years old - you have bedroom sex - when you have time - if you're not too tired.
By the time the kids are teens - sex consists of passing each other in the hall and lovingly saying 'screw you' to each other as you head in opposite directions.
By the time the kids are up and off to college, the wife is in menopause and the hubby has prostate problems - the spirit might be willing, but the equipment isn't always working.