If you haven't watched or aren't a fan of Supernatural, you can probably just skip this one because it won't mean much…
Paraphrased (because grammar and punctuation are apparently not a thing anymore) from something I ran across in a Supernatural posting on Tumblr. I'm on board with this.
Ya know I've read fics where Mary mistakenly thinks Dean and Cas have a secret relationship, but I have a much more specific vision that I wanna see. Mary and Cas bond. They are becoming real friends and she (in the most repressed way possible) alludes to being a lesbian and tells Cas that she's not a product of her generation; she knows things are different now and he doesn't have to hide anything. She's trying to tell him that she's okay with he and Dean being a couple. But Cas misunderstands it as her clocking him for being gay. He's kind of confused but mostly touched when he tells her "Thank you, Mary, i appreciate that," which she just takes as confirmation that Dean and Cas are together in secret. Later she says something similar to Dean which just short circuits his brain and he tells her "Uh. Cas? Cas and I aren't like that, Mom." to which she responds, "But he said you were." Dean replies with, "He…what exactly did Cas say?" and that's when Cas appears and eventually clears things up by saying "Oh I see. Mary. There's been a misunderstanding. I thought you were just referring to me being gay," and Dean turns to him and says, "You're what?!?"
I think Dean would be totally overwhelmed at finding out both his best friend and his mom were gay at the same time and this would freak him out in completely different ways; Cas being the most obvious. How did he not know his best friend was gay? Does this threaten already shaky heterosexual basis of their boy-best-friendship? Does Cas want to like, touch men?
Mary would be a whole other can of worms. Dean's understanding and view of his mother has already been rocked, but dear lord what does this mean for John? Did he know? Had she known? Had she always known? Was it a choice she made or was it God's plan? Dean inherited Mary's role. He's her mirror; he's a marycoded marygirl! Facing her sexuality, swirling in the back of his mind would be a just barely formed thought of "I got this from her too."
His first response would undoubtedly be angry disbelief and let's face it—Dean really doesn't have the capacity to conceive that it could be misconstrued as homophobia and not just the feeling of betrayal and confusion. I think it would be messy and would be one of those cases of Dean's emotions being so much and so impossible for him to examine that anger is the default response. I think he would grab his keys and take a drive to clear his head, leaving Mary and Cas alone to just look at each other wearing the same expressions of concern.
In the car, Sam calls and he picks up, "What? You calling to tell me you're gay too? Is everyone in this friggin family gay now?" and Sam just goes "… uh, no? I called because i have a…sorry…wait…who's gay?" and Dean simply hangs up on him and still has no idea why he's so angry.
