Dean and Sam were in a bar. I think Dean was flirting with some girl (prolly the waitress. xD) and Sam gets really jealous and kinda snappy. Dean figures out what is going on, and he whispers in Sam's ear that he is going to go back to the hotel, and he will wait for Sam. It takes 10 minutes for Sam to realize that Dean was using his seductive voice, so he rushes to the room. Sam briefly thought that Dean would be naked and spread on the bed, but instead Dean is sitting on the end of Sam's bed with his head in his hands(I think). Sam was going to say something flippant but stopped when he realized how serious it was to Dean, and how nervous Dean was. It was a Bottom!Dean story, and I know for sure that it is their first time together.

xD I found it! Its: Sine Qua Non by dreamlittelyo