Past!Dean who´s never been with a man before walking in on future!Dean and future!Cas having sex and being invited to join in.

After seeing how much his future self enjoys Cas´ cock, he decides he wants to try it too.

Past!Dean being all nervous about getting fucked for the first time, turning to Cas for reasurrance, but it´s future!Dean who wraps his arms around him from behind and whispers into his ear 

“Don´t worry, you´re safe, there´s no one in the whole universe who knows you better than the two of us.”