ON THE SELF, SEXUALITY, & RELATIONSHIPS:
NOTE: this is in coordination with @prodigil's sam.
dean feels a deep responsibility to the "idea" of what john winchest.er imparted to him of masculinity as represented by the hunting community. this is in large part due to the trauma inflicted by his mother's death & the tether he feels now connects him to his father ( & brother ) since he is all that is left to john & they are all that is left to him.
john raised dean to be a hunter, through & through. dean takes up this mantle as the "family business" without questioning or challenging as to whether this is the right thing for him. in his mind, he fits the order, not the other way around.
dean does not have a fully formed sense of self.
in season 1, we see this reflected as performative straightness & masculinity. he is showy, puts on a smile, uses bad pick-up lines, makes boob jokes, rough houses, etc. with his brother, & leers at women in public ( though rarely acts upon it ). in later seasons, he even lowers the timbre of his voice so that he sounds more masculine. this belies vulnerability that sits beneath the surface veneer of his behavior, but also serves to show that dean has wholly constructed his identity around what his father has told him he should be.
he routinely demonstrates anger in the early seasons that sam does not want to follow the same path or show their father the same blind faith that dean carries.
it is also shared that when sam decided to go to stanford & pursue freedom through the agency of choosing a path differently to dean, that dean was angry & even felt betrayed.
extending this further to gender identity & sexuality: dean absolutely accepts & loves his brother ( though he routinely demonstrates he does not know how to show that in an acceptable way - e.g. goading sam into physical fights, bringing to blows, mocking ) when @prodigil comes out as trans, but this challenges dean's own identity & role within the family.
he hasn't ever considered he could be anything other than what he is supposed to be: straight, cismale, & act like it. "real men" drink cheap beer, listen to classic rock, drive muscle cars, wear leather jackets. dean represses his attraction to other men, burying it beneath a layer of homophobia he has seen performed in the hunting community. he convinces himself this is liveable because he is attracted to women.
he also struggles with the fact that sam is able to reconcile his identity & come into his own. much like being able to go to stanford & claim a life outside of being a hunter, sam is able to fully come into his own, which dean resents. he feels as though with one queer person in the family, he is unable to take up more space with his own queerness. his self-loathing comes out in the season 1 episode "skin," where the shapeshifter calls himself a "freak" while wearing dean's personality, thoughts, & skin. he often does not consider his attractions, ignoring them, & if confronted, would say he does not feel them at all.
this repression also leads to resentment. he takes it out on sam often, sometimes deliberately, sometimes in microaggressions. this is not to say he does not love sam -- he does, & is actually more dependent on sam than sam is on him -- but he has not been raised in, nor has he developed appropriate coping mechanisms to fully abandon the more physical, toxic masculinity with which he has been surrounded into his early adulthood.