Based on Season 3, Episode 1 "The Magnificent Seven "
Prompt: "It's a part of you I never want to see, Dean" – Sam
So Sam knew he was lying, and he knew that making a joke out of it wasn't in any way going to make it more truthful.
If Sam was being completely honest, and he was trying not to be, Dean's 'Ahem' was something Sam really did want to see. Close up. And regularly.
In fact the trouser snake in question had already released its venom into Sam's mind years before when, as a mere teenager, Sam had walked in on his big brother jacking off. Not that Dean had seen of course – his eyes had been swimming in orgasm too much to notice anything, let alone his little brother with a boner; hard for Dean and Dean alone.
And on the subject of being honest, Sam wanted to taste Dean Jr. all over – especially knowing there was nothing at all junior about Dean's crown jewels.
Sam wanted to play an incestuous version of hunt-the-salami, wanted to tackle Dean's tackle. He wanted to make love with the love muscle and be poked by the pork sword.
However now would probably be a good time for lying again, because Dean is noticing that Sam is getting increasingly uncomfortable in the front seat of the Impala – and Sam really doesn't want to have to explain why he is turned on right now.
Things might just turn fugly.
Then again, as Dean pulls the car over and reaches across the front seat towards Sam's trousers, maybe not.