When someone says they just 'don't see it' or 'for them' they see Dean as straight, the instinct is to go back and forth with gifs or videos of Dean getting nervous around Dr. Sexy or Lawless, to send clips of Castiel's I love yous or Dean in Mourning and just...
That's not really "it"? because "IT™" is something I can't show in a gif. "IT™" is not a single instance of Dean getting flustered or watching a man's ass as he walks away.
There is too much queercoding, Too many pieces of set dec, Too much femme coding, Too many instances of hayes era film references, There are too many under-the-radar deep dive references to queer culture, There are too many 'mistaken for gay' jokes, There are too many 'not my type' 'I don't swing that way' gut reactions, There are too many story parallels to canon couples, There are too many 'interesting' scene transitions, There are too many bizarre episode recaps, There is Just. Too. Much.™ for it to have all been unintentional.
The character was based on queer people, both real and fictional.
Kripke might not have intentionally written Dean Winchester to be bisexual, but he is, & the writers who came after him clearly did.