P.E.N.I.S. - Post Ejaculatory Non-Interest Syndrome.
The post-coital complete lack of interest in the female companion that engulfs the majority of males as soon as their female conquest has brought them to climax
Girl: That was amazing, I love you so much.
Girl: Have you fallen asleep already?
Boy: Nope, just got a bit of P.E.N.I.S. Don't worry about it.