The sexual connotation of 'mark of the beast' involves heterosexual intercourse during the females menstrual period. Then, after gratuitous sex, the male phallus that is currently drenched in menstrual blood, marks an 'X' shape on the female, usually the forehead. She now has the mark of the beast.
"Oh, man, I would totally mark that beast!"
"I would boink my girlfriend, but I am afraid I would have to give her the mark of the beast, at least until saturday."