This is a fundamental law of manhood. Jacob's Law is that when referring to ones penis in a jokingly or flirtatious manner, you must portray it as smaller than it actually is. This means you never let a girl down once in the sack.

Originating from the Australian outback at Jacob's Creek hence the name

the whale tail is the shape formed when a g-string rides up high over a womans trousers

The scientific law maintaining that you can only do something not stupid for so long. Inferring that no matter how well things are going, something stupid is bound to happen.