Rick Warren, pastor of the Saddleback Church, became well known in the United States when his church hosted the Civil Forum on the Presidency during the 2008 Presidential Campaign. This event was something similar to the later presidential debates, and was the first occasion at which the two leading candidates, Barack Obama and John McCain, appeared together.
Subsequently, Warren was chosen to be the keynote speaker at the Martin Luther King, Jr. Annual Commemorative Service, and to give the invocation at Obama's inauguration. This caused quite a bit of controversy among LGBT activists and their allies, as Warren has publically opposed same-sex marriages. This high-profile controversy led to Stephen Colbert making a joke equating the term "saddleback" to an unmentionable homosexual act. A Savage Love reader wrote in to request the definition thereof, but Dan Savage was unable to find one. Thus, he opened a mailbox for suggestions, and nominated 7 finalists for public voting.
The eventual winner of this contest was a reference to the unfortunate phenomenon of misguided teenagers, after pledging in public to save their virginity until marriage, engaging in premarital anal sex. Stereotypically this activity is done without a condom (Bareback saddlebacking? How confusing!) as these poorly educated teenagers are forbidden to learn about such devices. As a consequence, saddlebacking could carry a higher disease transmission risk than premarital safe sex, along with a non-zero accidental pregnancy risk.