Back Door Man
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.
That is Betty's back door man she's with tonight.