To understand why the kiss happens, I think you have to understand a few things about Meg first.
Meg uses sex to torment people. A lot. She’s kissed Sam, she’s kissed Dean; I think she even kissed John at one point – and don’t forget, she once threatened to rape Jo. And that’s not counting all the times she’s straddled the Winchesters, grinding up against them, lobbing one sexually-charged barb after another their way.
There’s nothing sexual about any of this. This is violence, pure and simple, and sex is her weapon. She did all these things when the party in question was at his or her most vulnerable, tied up or otherwise held captive by her or her henchmen, and she did it as a way to show these humans their place and to demonstrate how much control she has over the situation. Meg’s kiss is a power play, a marginalization: She is sending the message that she is so unthreatened by whomever she’s kissing that she will take advantage of them however she likes.
It’s really unusual in pop media to see a female villain given such power – usually male villains are the only ones afforded the threat of sexual violence. It’s also unusual for writers to play that threat straight, meaning that female characters who do use sex as a weapon are rarely ever considered serious threats by the protagonists. A man who threatens rape is often an archvillain. A woman who threatens rape is considered kinky, at best.
This is one of the many reasons I love Meg, and that she exists. I’ve often argued that Supernatural serves up some of the most complex and fascinating female characters around (that they all tend to die isn’t the point, since in horror, everyone dies but the heroes), and Meg’s a great example. Meg is taken seriously. Her threat is real.
Which is why I think it was initially so jarring that Meg would ally herself with Team Free Will. While yeah, it makes total logical sense — on the lam from Crowley, Meg must find whatever friends she can, and hey, at least she knows where she stands with the Winchesters – it still feels a bit unsettling to the audience, almost like the writers are attempting to redeem her, or try to negate how vile she really was in past seasons.
I don’t think that’s what’s happening here. No, something else is going down in “Caged Heat”, and honestly, I think if Cas weren’t there, Meg wouldn’t have ever come knocking at the Winchesters’ door.
Even as far back as “Abandon All Hope”, Meg has been fascinated by Cas, maybe even a little attracted to him. And why not? He’s a fallen angel, just like Lucifer, and Lucifer is the cause that Meg has ordered her life by; she’s a true believer, a zealot even. So of course she’s going to be drawn to what she thinks is Lucifer 2.0, another angel who rebelled against God and Fell.
Note that she sticks around to taunt Cas even after Lucifer leaves, and of course this is the opportunity Cas needed to escape; he drops a pipe that knocks her into the holy fire – and into his arms.
Cas can’t smite her, however, and Meg seems delighted by this – she leans in close, drawing him near, teasing a kiss. This is what Meg does. She uses sex to torture captives, to remind them of their place. But never before have we seen her take so much time to kiss someone – or done so with such relish. If you didn’t know better, you’d almost think she was sincere, genuinely turned on by the angel and the circumstances.
Before she can seal the deal, of course, Cas tosses her on the fire and escapes, and that’s the last we see of the two together for a bit.
Fast-forward to “Caged Heat”. When Meg first joins Team Free Will to hunt for Crowley, she specifically calls Cas out and greets him, using the nickname she gave him when she had him trapped in the Holy Fire:
Meg: Remember me? I sure remember you, Clarence.
Cas: Why are we working with these abominations?
Meg: Keep talking dirty. Makes my meatsuit all dewy.
Meg has deployed another sexual barb — and yet, this time, it doesn’t feel like a weapon so much as defense mechanism. Cas insults her first, then she responds with the snappy comeback. Compare this to earlier in the episode, when her men take the Winchesters captive, and she straddles Dean’s lap, demanding that he “satisfy me, or I please myself.”
Honestly? I think Meg still has a bit of a crush on Cas, and is happy to get to pal around with him once more.
So. That kiss.
Before Meg draws Cas in, she actually makes a surprisingly heroic gesture, refusing to take Ruby’s knife from the Winchesters and instead offering to delay the Hellhounds, despite knowing exactly what Hellhounds can do to her. True, she’s sort of forced into it – as she can’t evacuate her meatsuit and flee, due to Crowley’s spell – but she still makes the stand, and still rises to the occasion where nobody else can. It’s remarkable, and the first time we’ve ever seen her do something anywhere close to heroic, and it explains, I think, what happens next.
Before everyone leaves, Meg pulls Cas in for a kiss, ostensibly so she can steal his angel blade – essentially asking for his help, while at the same time attempting to re-assert control and remind him of his place. But this time, the dominance play feels false. It feels more like of a hero asking for one last kiss from the damsel before making his heroic last stand – kind of like what transpired between Dean and Jo in the last episode Meg appeared in, “Abandon All Hope”.
They break apart. Then Cas shoves her against the wall and kisses her again, more fiercely this time, in a move he claims to have “learned from the pizza man”.
This of course references earlier in the episode, when Cas watches a porn under the assumption that this is a realistic portrayal of human love. (I think it’s safe to assume that this is probably the first time Cas has ever paid attention to how sex works before, despite watching humans have sex for many millennia.) What confuses him about it isn’t the sex, but that that the man keeps (apparently) hurting the woman. Is this what humans do, Cas wonders? Hurt the ones they love? Use sex as punishment?
With Meg, I think he finally gets it. Sexuality and love and romantic relationships – they’re all terribly complicated and messy, and often mixed with as much pain and hate as pleasure. And Meg is the one to make that lesson real.
When Meg kisses Cas, he’s deeply conflicted – Cas has always hated her, and yet, here she is offering to die to protect him and his favorite charges, and he can’t help but love her a bit for doing something so heroic. In fact, it’s kind of like the scenario he saw in the porn: violence and attraction, push and pull, love and hate. So he resolves the conflict by doing what the humans in the porn did, what he thinks any human would do, and kisses her back with interest.
I’d argue that just as the lessons Bobby learns in “Death’s Door” foreshadow what Dean must learn in TBAI, Cas’s kiss with Meg in “Caged Heat” foreshadows the ultimate emotional breakdown between him and Dean in “The Man Who Would Be King”/”Let It Bleed”. In “Caged Hurt”, Cas learns that love is as much pain as it is pleasure. In the final episodes of Season 6, this lesson is given practical application in the most heartbreaking ways.