What does key mean in porn?

What is key?

Another slang term, in a long list of alternative words, that refer to the male sex organ, the Penis.

Definition

Mainly used in straight parlances, in conjunction with keyhole, which refers to the woman's vagina, it is interesting to note, how many of these alternative words use terms that are symbolic of being important, such as a key.

The implication being that it is a feature that is vital to entering. A penis certainly fits that criteria, if you want to have sex.