When a girl loves many boys very much, she often partakes in a hefty amount of anal sex. Soon, the anus will become so large that it will actually converge with the vaginal cavity and become one. That is when a girl has developed a Vent. She will forever have one hole for all of her excretion needs.
"I was goin in on her for like two hours. I realized I couldn't finish because the Vent was so loose and floppy"