Key to the backdoor
It's a slang for the term anal. Meaning you put the male genitalia into the female's rectum.
E.G for key to the backdoor:
Male : I'll give you the key to the back-door baby!
Female : I'm not that dirty thank you... I don't like anal.
Male : I'll give you the key to the back-door baby!
Female : I'm not that dirty thank you... I don't like anal.