One thing I find interesting is that Sam and Dean who are as we all know very open about their sex lives with each other…with one exception. (I’m only talking kripke era) Ruby. When Sam is telling Dean about him and Ruby getting together it is the only time in five seasons where Dean cuts Sam off not wanting to hear more. Ruby who is the only other character to call Sam “Sammy” and not be told “only he (Dean) can cal me that.”
Ruby is the biggest relationship other than John to impact the boys. She is the wedge, the only person to be jealous of. Sam can hookup with girls on hunts but only the once. This is more. And she knows all about how dangerous their lives are more so then they do at the time. Creating jealousy and suspicion. She takes care of Sam and that’s Deans job. She never hated his freak qualities encouraged them when Dean shamed them (out of fear).
She nurtures Sams most hated aspects of himself. But also when he feels the strongest. When he can use this curse to help people. If he helps enough people will it scrub his blood clean?
so okay i'm working on 7-month old memory with ruby because i'm still only on season 2 in my re-rewatch but let's compare ruby and meg (i feel inadequate because i have such extensive notes about meg now but nothing about ruby lol).
they both occupy the same narrative role: to influence sam down the path of his destiny. they flirt with him, get close to him, say what he wants to hear, all to bring sam closer to his fate. they do this through manipulation so that sam will embrace his path and stop running for it.
dean is the diametric opposition to sam's fate, (narratively) the only person in the world who can stop him. this goes the opposite way as well: their choices in the story are between their fate and their brother, and it's only by choosing their brother that they ever manage to avoid their destinies.
so what both meg and ruby do is pull sam away from dean. this is subtle, and they go about it in different ways. meg is aggressive and encourages sam to hate dean, turn away from him. this is an appropriate tactic given the context in which she meets sam, because he and dean are fighting and separated and she's trying to embolden sam to strike out on his own and leave dean behind.
ruby on the other hand can't use that same strategy. they've already entered into a codependent relationship when she shows up, and if the choice is between literally anything else and his brother, sam is going to choose his brother. so ruby uses that against him and becomes dean: and not just dean, but an idealized version of him. the problem, of course, is that this idealized version of dean is the version of dean which is enabling sam to pursue his fate. sam wants dean to accept him and love him warts and all, so ruby does that in dean's place—and that's the danger of it all. she makes his fate out to be desirable and alluring, using dean's voice all the while to seduce him. ruby is not dean: she's just using his likeness to blind sam and manipulate him into fulfilling his destiny. choosing ruby isn't the same as choosing dean, and therefore in choosing ruby sam is unable to avoid destiny.
by all accounts it makes sense for dean to hate her: he not only has legitimate reasons not to trust her, but she's also manipulating sam by impersonating him. she's trying to take his place, and obviously that is going to make his fear of abandonment flare up.
but on the other hand, dean reacts completely differently with meg, even though she too embodies dean's fear of abandonment and represents sam leaving him. instead of being repulsed by her, dean pressures sam to fuck her and even after they figure out she's bad news, dean continues to tease him about this fabricated love affair they had going on. dean's reaction here is understandable too—he's displacing his fear and sense of inadequacy via sexual commentary because he believes sam is going to leave him for a girl.
both reactions are understandable given dean's psyche, but what makes them different? my current assumption is that dean simply learned from meg's example. she was obviously deeply traumatizing to both sam and dean in different ways, and meg is also the origin of dean's firmly held belief that demons are liars. so he's not quick to trust ruby, and she doesn't give him many reasons to trust her, at least when put against her red flags. monsters (especially demons) are also generally repulsive to dean, exacerbated over the years of battling against their respective fates. dean is disgusted by ruby, so he wouldn't want to know what she and sam are up to in bed.
on the other hand, ruby comes to embody just how thin the tightrope sam's walking actually is. sam shouldn't trust her, and he doesn't at first, but this is his fate we're talking about here. she worms her way under his skin and convinces him it's all okay. dean is probably not so blindsided by her because this isn't his fate—he's not faced with the allure of this path the way sam is. his direction is different, and he's blinded in different ways that sam can see straight through just as well.
it's kind of like how dean is exempted from the revenge arc in season 1: he stands on the outside of the conflict because he is in fact its opposing force. where sam is blinded by his fate, dean can see things for what they are and (try to) guide him back to the family. sam does the same thing for dean too because as i said, choosing the brother over everything else is the only way for them to avoid their destinies.
so way too long story short i think that's probably why ruby disgusts dean so much: she disgusts him, and he can see past her facade because he's not drawn in by fate the way sam is. he has no reason to trust a demon and no extenuating circumstances that would sway him to consider it, either. ironically, this just fuels the conflict and drives the wedge between sam and dean deeper because ruby's manipulation is just so complete and so perfect. there was no correct choice: sam was doomed from the start and there was nothing dean could do to stop it.