The first time that Dean saw them alone together, it was completely by accident. He had gone to bed fairly early, but instead of getting extra sleep he just woke a couple hours later. They had probably assumed he would be out for the night, and Sam hadn't bothered to make sure his bedroom door was shut.
He got up to grab a snack, and as he passed his brother's room he noticed that the door was open. Through the opening he could clearly see Sam lounging in bed with a book in hand, and an angel pressed to his side.
It wasn't as if they hadn't told him that things were going on between them, so he wasn't surprised that Castiel was in his brother's room. What he was surprised about was the obvious level of affection between the two of them.
He didn't really know what he thought went on when he wasn't around. As a matter of fact, he may have avoided thinking of it altogether since they'd told him. He'd been in a kind of live and let live haze about the whole thing.
It was weird, to him, that his brother had grown so attached to the angel. It was weird to think that Castiel was even interested in that kind of thing. It was super weird to think that his baby brother was having gay sex with an angel. A combination of weird that had sent Dean into a solid denial about the whole thing. It was don't ask, don't tell around the bunker.
Seeing them now made the hunter rethink his stance on that.
As far back as Dean could remember, Sam had always read himself to sleep. He always had a book handy, and even invested in tiny lights that attached to the pages so that he could read in their hotel rooms without disturbing Dean's sleep. Seeing his little brother with a novel in hand was nothing new. It was the addition of the angel's head on his chest that was new. Castiel had his eyes closed and his features were so relaxed that he could have been sleeping. Angels didn't sleep though, and Cas was back to nearly full power.
As if to prove that point Sam whispered into the silence of the room, and his question was immediately answered, with the angel's relaxed facial expression unchanging.
"So, does Manny get out of this trap? Or are they finally getting rid of him?"
"I suppose you will have to finish the book and find out."
Sam huffed an amused breath at that.
"Since when have you cared about spoilers?"
"Dean said that it is rude to tell someone how fictional stories will end, even if they ask you to."
Sam's mouth stretched into a soft, amused smile and he looked down at the mop of dark hair on his chest.
"Of course he did." And he turned his attention right back to his book.
It was such a simple scene.
It wasn't the forced interaction of two people stuck in a room together. The uncomfortable conversation with someone you had very little in common with. It wasn't that 'we just had sex, so maybe we should pretend to talk' scene that Dean usually had in the middle of the night. This was comfortable, as if it had happened a million times before. As if his brother fell asleep every night with the angel wrapped around him. It was closer to the nights he'd spent with Lisa, back when things were good between them. After they had already learned all the quirks and irritations of the other and found a harmony in the household.
Dean felt like the biggest asshole on the planet now.
He had inadvertently forced them to continue to keep their affection a secret. Not by telling them they should, but by effectively ignoring what they had been trying to tell him.
They hadn't told him about it out of guilt that they were having some kind of affair behind his back. They hadn't confessed to him out of some kind of duty to inform him of their actions. They'd told him in the hopes that he would be able to accept them. They told him so that they would no longer have to feel the need to keep it hidden. They probably wanted to act like they were together in more places than just their bedroom. Hell, they probably wanted to be able to cuddle on the couch in their own damn living space.
How could Dean have been so blind? Of course they wanted his blessing on their relationship. He was possibly the most important person in both of their lives. Of course they wanted to make sure he wouldn't be uncomfortable if they showed their affection outside of the bedroom.
He'd been so thrown by the information, and so quick to build a wall around it. He hadn't even realized what the point of it had been.
He made his way back to his own room, all thoughts of midnight snacks gone from his mind. He had several hours to work out how to talk to them. How to tell them that he could accept whatever was going on between them. How to tell them he was happy for them.
He wouldn't sleep tonight because he needed to talk to them by morning. He didn't want their love to be stuck behind closed doors for even one more day.