Disclaimer: Supernatural belongs to Eric Kripke and WB. This was just written for entertainment
Sam loved blow jobs. He was a guy, of course he loved them. His little stint of semi-celibacy (his relationship with his right hand didn't matter in this case and was totally private) didn't change that. So, seeing a nice mouth gave him ideas and more material for his wanks in the shower. And then came the fateful day when he realized that one of the most indecently hot and sinful mouths belonged to his big brother.
Bummer.
He did what every self-respecting Winchester would do in this scenario: denial and repression. And that should have been the end of it.
Don't think of a hippopotamus. That was the sentence Sam remembered and in front of his inner eye a familiar pink Disney cartoon hippo danced in a tutu.
It was the same with these damn lips. Everything that had anything to do with mouths and lips reminded him of Dean's: Eating, talking, he always flashed back to this one specific version. And he started to watch said mouth every occasion he got away with it.
When Dean flirted with women, when he hustled someone, when he ate and drank, when he sang along to one of his tapes in the car, when he interviewed somebody for their current case or calmed a victim. And that really reminded him of blow jobs. Because his brother Dean was so orally fixated it wasn't even funny. Not in the perverted way that he stared at his brothers mouth like Sam, no, in the way that whenever he wasn't eating or making out with some woman, he was licking his lips.
It certainly explained one of the things that made him so appealing to women. How had Sam not seen this years before, that unconscious behavior was nothing new. The realization changed at least one thing: Sam needed longer in the shower.
"You know, I wondered if you would say anything."
"Hhm?"
"You've watched me like a creepy stalker dude for months. I expected you to say something by now."
Oh no, this was not good, this was seriously not good. Sam wondered how he got out of this, how he could manage to keep the depth of his depravity under wraps. "I'm... I don't know what you mean."
Dean arched one eyebrow. "I'm not blind, Sam. I know you want me on your knees and blowing you. What would you say if I enjoyed you on your hand and knees and pounding into you till you are so sore you have problems sitting?" His expression was a cross between flirting and mocking and Sam wondered how that could look so hot. "Or do you prefer if we doubled your shower time again? Because you surely don't get hints." And after a second of nothing, he provocatively and deliberately... licked his lips.