(Disclaimer: I just want to acknowledge that I do not own Supernatural or the characters. The situations, however, are my own.)
Castiel has always been on the receiving end of penetration of his and Dean's relationship.
Even though some would argue he should have been the one putting forth penetration, being an almighty angel of the lord and what not.
But Dean wasn't handling his feelings well, and Castiel knew a big part of that had to do with John Winchester.
Even now, as Dean roughly pounded into Castiel, Dean couldn't look at him. He always made Castiel face away from him, but Castiel knew Dean had his eyes closed anyway. It's easier to pretend that way.
And even if Castiel didn't know all these things then, afterwards, when they were both spent, Dean would curl up into a position that made him seem much younger than he was, and his shoulders would shake with sobs and you could barely hear him whispering how sorry he was for being so stupid and disgusting.
After first, he would get angry whenever Castiel tried to touch him. Now he still tenses, but at least he let's Castiel wrap his arms around him comfortingly.
So yes, as an almighty angel of the lord, Castiel is a bottom. Because Dean needs this. Needs to know that he is still somewhat manly, even though if his father were here he would most definitely disagree; he needs to know there's still one thing he will always be in control of.
Dean still isn't open to the relationship outside of the bedroom; late at night, hours after everyone had long went to sleep. But they're making progress, and that's all Castiel cares about.
Because Dean is worth his patience. Castiel has lived since the creation of time, and he can wait a few years for Dean to stop paying attention to the voice in the back of his mind that whispers what if your daddy found out?
So Castiel will take every rough pounding Dean throws his way, because he knows Dean is broken and confused. Castiel is an angel of the lord, and can handle worse beatings.
Besides, it's worth it when, in the wee hours of the morning, before everyone wakes up, when Dean wakes Castiel up by gently kissing all the bite marks left from holding in sounds and finger-shaped bruises left from hips gripped too tightly.
And then Dean asks Castiel to leave, and promises that tonight will be better. Castiel knows it won't be, but he knows that Dean doesn't, and is legitimately trying.
One day things will get better. One day the last of John Winchester's influence will be swept away by the love and affection transferred from angel to human.
But until then, Castiel ignores the tear drops he can feel on his back; ignores the fact that Dean's eyes are closed and his head is turned away in shame, trying to imagine anyone else but Castiel; ignores the fact that John Winchester's voice is probably echoing in Dean's head right now, when did you become such a queer? Wait till I get home, boy.
Until then, Castiel takes the beating.
