I actually kind of love this scene, and I think for the most part it speaks for itself… so just don’t understand how Destiel shippers are upset about it! I read it as entirely supportive of a Dean and Cas relationship. And here’s why:
Cas is wearing a somewhat cheeky look when he brings up April because he knows it will perturb Dean. And the first thing that Dean remembers about her is that Cas had sex with her. So, Cas knows he’s struck a chord just like he had been predicting and (I’d say somewhat mockingly, yet acting quite innocently) says “Yeah, and you stabbed.” (Like that’s another thing you might remember about her Dean, if you cared less about who I was sleeping with.) And Dean picks up on the fact that Cas is taking a jab at him, hence the open-mouthed reaction and the glance at Sam (to see if he’s picking up on what Cas is really getting at). “She was hot,” Dean says, trying to lead the conversation back to safe territory. And I mean, I guess you can see it differently but Cas isn’t exactly hiding his eye roll when he responds, “So hot.” Because all Cas knows about “love” and relationships is what he’s learned from Dean, and although looks may be among the most important things to Dean, Cas mostly liked her because she was “very nice.” And the next clip is Dean realizing just how far Cas exists outside and above the norms of our society. Cas liked April first and foremost for the person she was (//well who she was pretending to be//) rather than reasons that are normally highly valued: looks, conformity, etc. Cas doesn’t care about societal norms, I mean come on, he's utterly indifferent to sexual orientation. And Dean is very caught up in this realization, jerking back into the moment and casting a wary glance at Sam before a dazed expression crosses his face once again.
And Sam! There is no better expression for watching your brother realize that it’s perfectly fine to be head-over-heels in love with the guy sitting next to him. As if the whole exchange wasn’t incriminating enough, it’s Sam’s reactions to the whole situation that solidify my opinion of this scene. I mean, he’s playing the awkward third-wheel perfectly! What other way can it be described?
As for that last gif… “Not every hook up’s perfect.” Need I elaborate? You can read it as a reminder or a promise (**that smile, though**), but one thing’s for sure… Dean and Cas are desperately, hopelessly, dewy-eyed, rosy-cheeked, perfectly in love.