It's AU because Sam and Lucifer are together as a happy couple.
Sam makes a bet with Lucifer that he wouldn't have fallen for him if he didn't know who he (Lucifer) was (I think). Lucifer says that he is iressistible and goes back in time to before the whole Lucifer story arc...Sam and Dean are investigating some incubus at a strip club (Disney Princesses maybe?) and Sam wanders around and finds a door to another part of the club where Lucifer is dancing dressed up as a stereotypical devil (with horns and a tail). Sam is really, really attracted to him and they end up having sex with the horns and tall. Lucifer then blurs Sam's memory and goes back to his Sam and unblurs the memory for him.
I can't believe I remember so much of the story and can't find it!
Any help or suggestions are greatly appreciated.