the man that some ones wife is seein on the side. usualy came to the back door to avoid any other house wives seein him.

more background:
because the man she's cheating with would often use her "back door" for sex, have sex anally, so that the husband wouldn't detect she's had sex with someone else (if he had sex with her later). Also, for the woman, to avoid pregnancy by someone other than her husband. (and thus, avoid getting caught cheating when the baby didn't look like, or was a different color than, the husband.

One who perfers to insert their penis into an anus

A song by the Doors, referring to the meanings of the other definitions of this phrase.

A song written by Willie Dixon.
Covered by "The Doors"
It is about anal sex.