Benny rapes Cas, and then convinces Cas to not tell Dean. He says Dean would blame Castiel for what happened. Believing Benny, Castiel tries to hide what happened (it could keep happening, too) until Dean finds out (either Cas accidentally tells him, or the rapes are recurring and Dean sees it happen). Comfort ensues.

The extent to which Dean and Cas are in a relationship, or just friends, is up to the author. Could happen during Purgatory or after.