Dean teaches Sam to play pool at some bar. And people know they're brothers. Maybe they came with John who is now wasting away at the counter.
And Sam beds over the pool table, looking behind at Dean with his big innocent doe eyes. Dean bends over him, saying: "here, let me show you", while he rubs his dick on Sam's little ass.
And Sam is still so young, he doesn't expect that even though he was the one to incite Dean. He doesn't know how to react, flushing red and all shy.
And people look at them weirded out because they know they're brothers. And maybe some find it kind of hot.
And then they find themselves in the bathroom, maybe Sam dragging Dean away cause he's too embarrassed of being looked at while like that, or maybe Dean drags Sam away to get a little privacy to do even more unspeakable stuff.
And when they return it's clear what happened. Sam is even more red, their lips are red and puffy and they both have new red marks on their necks and Dean's zip is still down.
And John doesn't realize because he's completely drunk. He sees them and he shouts at them to come close cause they're leaving.