Basically it all comes down to the fact that this is totally normal.

They're best friends, they've been together all their life, now they're even married and yes, Shawn remembers that he climbed into Gus' bed last night. This is not awkward, actually it's nice and there are so many arguments why the fingers are moving lazily over Shawn's skin. It's just basic affection and who knows, maybe those crazy IRS dudes even installed secret cameras! But even if they didn't, this is nothing out of the ordinary. Weird, but nothing out of the ordinary.

Still, it doesn't explain the heat that creeps up to his face and makes him blush furiously.