This New Condition of Living by sevenfists
Dean teaches Sam about sex with porn. Sam/OFC, Sam/Jess, Sam/Dean, but really it's all Sam/Dean, and there is this one exchange between them that is seriously THE BEST THING EVER and I can't tell you what it is because I don't want to spoil it, so you will just have to go read it now and wriggle with glee like I did.
Also you should go add prompts for the Evil Sam Ficathon. Just reading the comments is delighting me. ♥