The anus, especially in reference to anal sex.
Origin: picture the female perineum in the missionary position, and note the orientation of the vulva relative to the anus. Observce the similarities with the typical orientation of the front door and the cat flap in a house.
Other parallels regarding size and ease of human access further strengthen this metaphor.
"I've got the painters in this week, but you can go in through the cat flap if you like"
Prices shown in USD.