One of my favorite things about this episode was how embarrassed Dean gets when Cas catches a glimpse of the “Busty Asian Beauties” website pulled up on his laptop.
Because it’s the scenes like this - Dean being uncomfortable with Cas watching him sleep, Dean being uncomfortable when Cas sees his porn - that show us that what Dean feels for Cas is galaxies away from being the same sort of brotherly affection he has for Sam.
Dean has no qualms sleeping in front of Sam, whether in the Impala or in a motel room. Dean has no problem trading dirty jokes with Sam or giving him skin mags for Christmas.
But here, Dean covers the laptop with his hand so Cas can’t see. It’s the extreme mortification of anyone who’s had their crush find the centerfold on his display in their room, or the stack of Playboys under the bed.
But more than that, this scene shows that Dean still doesn’t want to think of Cas in a sexual way. Because of course Dean remembers what happened the last time Cas watched porn - he got a boner and then made out with Meg. And here’s Cas, and there’s porn. And he and Dean are aloooooooooooone.
But why does Dean want to keep himself from thinking of Cas that way? It probably comes down to the simple fact that Dean still thinks of Cas as an angel. And, as I have read speculated in several metas, Dean might fear that if he approaches Cas in a sexual manner, Cas will leave - he uses sexual references like “Blow me, Cas!” and “Last time…I got laid” to push Cas away in late season five.
And more than that - Dean covering the laptop is also an attempt to shield Cas. Even after all the wars Cas has fought and all the blood Cas had spilled, Dean still wants to think of Cas as an innocent, which is where all those “baby in a trenchcoat” jokes came from in season six.