He knows

What happens when the small-town homophobic teens bet Dean Winchester, a struggling peer, to date the town gay punching bag Castiel Novak?

Dean knows he's not straight but he didn't know that Cas's eyes are that blue up close.

Castiel knows Deans different from the rest of them but he didn't know Dean would be anything but straight.

Though, the real kicker is that Castiel Novak knows it's a bet.

-Destiel short story