"Because it's a dude." Dean said quickly, like the reason was obvious. "What am I supposed to do with a dude?" Dean asked, and it was only after a moment or so of thought that he realized Cas probably would never get why that was weird. He also wasn't that big of an idiot, he knew what two dudes did. He'd even made friends with a couple of hunters who were just a couple of dudes...doing things together that he had pointedly never thought about.
He had to mentally give them props for holding it together, though.
Backpedalling in his head, Dean took a deep breath, blinking a few times and staring at the ceiling like it might actually start talking to him. "Dudes...don't ask me out, Cas." Dean was no stranger to attention, a nice smile and a friendly disposition usually got him plenty of it, but he could probably count the number of dudes who had actually made some kind of move on him on one hand. Then again, he'd been tricked by that one guy... not important.
"It's not my custom...and I can't be a dick and back out of it if it is...but I don't want to go on a date..." He closed his eyes, trying to calm himself, but some little crazed part of his brain could just imagine Sam laughing about this. That wasn't helping him at all. "It's just bowling. It's not." He swallowed, trying to get himself to not get insanely flustered again. "It's not a date." It couldn't be.