When watching Supernatural 4.09 and 4.10.

Ruby is a demon. Ruby doesn’t weat a bra, because she’s a bad girl, and she sleeps with Sam, therefore Sam likes sleeping with demons.

Anna is an angel. She wears a white bra becasue she is pure, and she sleeps with Dean, therefore Dean likes sleeping with Angels.

Conclusion:

Dean/Cas is inevitable because once you’ve slept with an angel it’s never the same again and also Castiel wears a bra.

That is all.