For one thing, such a relationship would make sense in canon, as Dean was tortured and became a torturer in Hell, and Castiel raised him up from it. Castiel knows Dean inside and out and forgives him. Real BDSM (unlike in certain not-researched popular novels *cough*) is about complete trust and consent between partners, and could prove quite comforting to Dean, who needs to be submissive or dominate as the situation demands. Dean doesn’t have to worry about harming Cas, and Castiel can heal Dean.
And we all know Cas loves Dean, he’s died enough times to prove it, so it’s not a relationship of non-equals (with a harlequin-romance on steroids trope of hapless virgin and stalkery obsessive partner), imho, Cas doesn’t need physical release (as an angel he is as Dean says, junkless) but wants to help Dean.
forgive the silly song, but nice clips, eh?