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.
Often times referred to as anal sex, "back door man" also means to enter the vagina from the back side so you can pleasure her breasts and clitoris more freely with your hands.
A back door man is a man who likes to bang a woman in the ass. He’s the “get bent” type of guy.
“Back door man” expression was used in the 1930s and the 1960s psychedelic rock band The Doors adopted it where Jim Morrison uses the song’s title and context to define a man who has anal sex with his woman.