Ok but listen. The demons? They know about Dean. (and Cas but just a sec.) They know about Dean because of what’s “written on demon bathroom walls.” Because of the “summer of love” with Crowley. Because when he’s his most stripped-down, uninhibited self (when he was in hell) they see him being not straight and not giving a fuck.
So when demon fanboy #1, who just happens to be associated with both LUST and DISCOVERY swoops in and tells Dean he’s “gorgeous” he knows what he’s doing. He’s hoping…and he’s testing…and it will definitely NOT work while he’s wearing Dean’s son, which I think you can see him being a little disappointed about after he asks “so who was he?” and gets his answer.
His further observations that Dads are fighting (ooohh awkward!) tells you that, yeah, he reads the demon newspapers and, as that classic Tumblr post suggests, the tabloid covers really are all about Dean and Cas. Remember the first episode of Season 14? “Joined at the…everything”? They. all. know.
And they all think that those two must be banging because, I mean, c’mon! They know Dean’s reputation. They see that they’re fucking married. How could he not be hitting that? (The angels know too. It’s why they are so pissed and judgmental about Cas.)
So when the writers choose to have demon fanboy #1 show up and flirt with Dean and unintentionally sow more discord between him and his husband by wearing the dead son they’re in a fight about…that signals a major plot arc for the season all rolled up into one.
The demons know about Dean and taunt him by coming thisclose to revealing it all the time. The object of DeanCas discord is that very same demon because he’s possessing Jack. I know others have said this at greater length and in a more nuanced way but DeanCas discord + secret sexuality + Cas leaving = Dean in crisis = revelations. Soon.