When two people are having buttsex, the man on top pulls out right before cumming and instead runs his penis all over the other persons unfortunate back, leaving a trail of shit and semen. He then proceeds to rub it all in, leaving their backside an unfortunately darker shade.
Phil: Dude, have you seen Chad Warden? Is he mexican or something?
Dave: Oh no dude, he just had a few Texas Suntans.
Phil: Huh, I thought I smelled cum and shit when he walked by.