dean gets aggressively sexual in his comments about meg in 1.16 shadow immediately after finding out that sam met her while they were separated in 1.11 scarecrow, in a way that he isn't with sam's other love interest in the first half of season 1, lori in 1.07 hook man. it's rather more reminiscent of how he acted during his brief moments with jessica.
his first interaction with her is to "compliment" her boobs, right in front of sam—it feels as though he's trying to stake a claim, put her down (in her place, perhaps), assert a sense of dominance over her. in shadow, this weird, sexual fixation surfaces again with meg, and dean can't stop pressuring sam to fuck her or insinuating that sam has already fucked her.
the only noticeable difference between lori, jessica, and meg is that dean was present for the formation of the relationship with lori. but meg—sam never even mentioned her. they met when dean wasn't around, when sam was living his own life. on top of that, meg accused dean of treating sam like "luggage" and dragging him around—implying that sam would leave if he could.
comparing lori and meg, it comes off then that dean is feeling threatened here, abandoned yet again by his brother, and indeed the insults meg slings at him in the beginning of the episode resurface during their emotional conversation when sam says he plans on going back to school. meg has come to represent sam leaving him, and throughout the episode he overcompensates for his insecurity by channeling it all into this invasive sexual commentary.
what this ends up implying is that dean wants to fuck his brother.
dean’s fear of abandonment presents as such that he always feels sam is one wrong move from leaving him and going off on his own. it’s a deeply anxious attachment, and he ends up overanalyzing every interaction they have and misinterpreting any attempt on sam's part to put or maintain distance between them as total, unambiguous abandonment.
and in scarecrow, sam really did abandon him by splitting off from dean and making his way to california (or at least, that was the intent). scarecrow is the realization of dean's core fears: sam will eventually leave him, sam wants to leave him, and in the end they’re only together out of convenience.
and now dean finds out that sam met a girl while they were separated.
the conclusion he must have reached is that sam will inevitably leave him in favor of a girl: at stanford he had jessica, here he has meg. both times he and sam were apart, sam found a (pretty, blonde) girl to get close to. the girl therefore comes to represent sam's abandonment, or rather the goal of sam's abandonment: to find what dean can't give him.
because if sam secretly doesn't want to be around dean, then dean isn't providing an environment or a relationship that sam would want to be in. sam wants to leave, so dean is somehow inadequate.
that feeling of inadequacy, both in the pilot and in shadow, manifests through a sexual lens, just as sam's abandonment is represented by the girl. by putting these examples together, dean is seeing the impetus of sam's abandonment as something sexual: sam wants sex with girls (and a stable relationship beyond one-night stands), so he is leaving dean to find that. this connection is so strong to him that he's ignoring sam’s actual feelings and displacing his own biased perspective onto sam.
dean cannot fathom sam interacting with meg platonically, and this is rooted in his feelings of inadequacy and fear of abandonment. dean believes he can't satisfy sam's needs to an extent that will prevent sam from leaving him, which according to dean are sexual in nature. he can't (or won't) put out for sam, so sam is going to leave him. and to hide these fears (from sam, from himself), he gets sexually aggressive about the women who threaten to steal (or succeed in stealing) sam away from him.
or: dean won’t fuck his brother, so sam fucked off and found a girl who will.



















