When you and your partner are having sex, one partner gets into doggy style receiving position while the other partner gets on top of the other partner as a person might mount a horse. They then continue to defecate upon their partner's back and slide the feces around all over the partner's back. The slop upon their back and the other partner's genital area resembles a saddle.
Katie: I was having sex with Joe last night, and went into doggy position when suddenly he got on top of me and pulled a dirty saddleback!
Sally: Oh my gawd! I wish Stan would do that. So hot!!
Saddlebacking - the term for the phenomenon of Christian teens engaging in unprotected anal sex in order to preserve their virginities. Named after Saddleback Churh, home of the Reverend Rick Warren, who is a proponent of abstinence education, the "sex ed" that has convinced so many Christian girls and boys that buttfucking isn't actually sex.
Saddlebacking - After attending the Purity Ball, Heather and Bill saddlebacked all night because she's saving herself for marriage.
Sheesh! I have seen this phrase misused a few times now (even in Roger Mellie's Profanisaurus!). Let me clear things up:
Saddlebags is a word to describe the bulging areas of fat on the upper outer thighs of ladies, in a way that resembles saddle bags being carries on a horse.
The expression was used in Ace Ventura: Pet Detective. I can't remember exactly how though. It was something along the lines of:
Melissa Robinson claims she isn't fat. Ace says something along the lines of "yeh, whatever you say, saddlebags!"