Any form of sex is physical activity above the basal metabolic rate, (BMR). This includes masturbation. BMR = metabolism at rest or the "rock bottom" fuel requirement to sustain life! Aka, amount of calories needed to sustain a physical body at rest or death. Without "fuel" even a dead or resting body will eat itself into a thinner being. Without the yolk or egg white "provided" a chicken would have a much more difficult time developing from an embryo to a rooster!