When a woman has just completed her period and wants a little extra time before jumping back in the sack again.
After a week of abstaining from sex while my girlfriend had her period, I was flabbergasted when she told me that she was still on an extended vaginacation.
Having sex with the person you're in a relationship with, and then having sex with your side piece. Later bringing it full circle back to your original partner with the possibility of transmitting an STD.
I banged my wife yesterday. Today, I banged my side piece, and then had to bang my wife again, thereby causing cross-vagination.