That it wasn't John's idea is about the only saving grace in all of it.
John and Dean in a bar, and John overhears two strangers talking about all sorts of things they would like to do to his son. He listens to them talking about his lips and what they'd feel like, about how his face would be red as he'd cry (John corrects in his mind that it's not like that—Dean's what other people would call a "pretty crier"). He can't quite keep the images out of his head, his imagination adjusting the details that those strangers get wrong, until he can get it perfect: Dean on his knees, one of those lone tears running down his face, shining eyes looking greener under a certain light.
It's not his idea—it's just, he's had a few drinks, and he's not a saint, of course he's going to imagine things when people are saying shit like that. And yeah, Dean's his sixteen year old son, but he is—well, John has eyes.
Then the guys say something about how good Dean would look ruined, and that's what make John leave. It's that or put a bullet into their heads.
John guides Dean out of the bar with a heavy hand on the small of his back and he can't help but glance back at those strangers. Dean and him—they aren't like that; John would never do it, even if a treacherous voice whispers to him that, really, would it fuck up the boy all that much worse, after everything? But he likes those men seeing his claim on Dean, likes the envy he can see in their expressions. It's almost like vicariously living it all in their fantasies—it's just unreal enough that he can deny it all.
And then Dean falls asleep on the passenger seat, his face turned to John, mouth slightly parted, and John can't help but understand the desire to ruin something so beautiful. It's a dangerous thing, to think about how real it could be—some part of him refuses to acknowledge how easy it would be, how accommodating and careful and good Dean would be for him. He could park the car and tell Dean to do whatever. He wills himself to believe that Dean wouldn't—that he'd shout and fight and take Sam and run away. Maybe he'd kill John for trying that.
That idea has to be enough to make him push through—just through this night, because come morning the alcohol will be gone, and these thoughts will be nothing but a memory.
(The thoughts don't go away, though. It wasn't his idea, but now that it was put into his mind...)
