A boner downer is a term which is used to express not only the frustration of being turned off, but also the actuality of losing a boner. A boner downer may occur when something someone says turns your boner or any potential boner limp instantly.
Guy 1: Man you should have been in the child development studies class yesterday! There was this smokin' hot babe in tight yoga pants and you could see right through them!
Guy 2: Shit I can't believe I missed that, I slept through my alarm.
Guy 1: Yeah she was so hot...but then the speaker started talking about teen pregnancy and it was a total boner downer!