A random thought just crossed my mind. Remember in season 4 when they first found the Winchester gospels and everyone was freaking out about Sam having sex with Lilith? And the reason for the sex turned out to be that Sam made a deal with her but as a powerful demon that meant sealing the deal needed more than a kiss?? Remember??
Yet now they are constantly making deals with the KING of hell (who surely gets the status of ‘most powerful demon’).
Therefore, taking Deans illusive shorts as a starting point (he NEVER wears shorts after all) and going by the shows own canon, we can conclude that you must ALWAYS have sex with a powerful demon in order to seal the deal.
This means Dean has been banging Crowley since season 8. (And therefore so was Cas in season 6).
100% evidence right there. The shows own CANON says so. So there.