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."