“The penis is the dipstick of the body's health,” says Dr. Harry Fisch, a urologist in NYC and author of multiple books on health and sexuality including Size Matters: The Hard Facts of Male Sexuality That Every Woman Should Know. “The equation is simple. If you’re feeling low energy, you’re not going to want to have sex. When you’re having sex, it’s an energy release and you tend to be happier and have more energy to do things you like.” Including, more sex. So guys, make the first move.