When your doing anal and the girl queefs on your balls, causing them to sway back and forth from the air flow.
The air gap between a ladies inner thigh.
Damn, bro your girlfriend has a nice clam vent. She must work out a lot. I bet her pussy don't stank at all.
The act of having sexual intercourse in a ventilation duct.
I went vent thumping with my girl last night at the theater