Bisexual in the Subtext since S1
If you ever wonder whether some members of the Supernatural team were writing Dean as a (closeted, even unto himself) bisexual character, in the show’s subtext long before the arrival of Castiel, remember to rewatch 1x12 (Faith) by Sera Gamble and Raelle Tucker. Still one of my favourite episodes.
Dean’s heart has been severely damaged by electrocution and he is dying. The heart is symbolic of the essence of a person e.g. “I loved him with all my heart”. Gamble and Tucker could have chosen to make Dean terminal in any number of ways, BUT they chose his heart.
Sam takes Dean to a faith healer and he is cured - his heart is repaired. A young man called Marshall Hall dies of a heart attack at exactly the same moment.
Again, Gamble and Tucker could have killed Marshall Hall any way they liked, and the narrative would still have worked on a textual level. BUT they chose his heart.
Later Sam and Dean discover that the healer’s wife has been controlling a Reaper to swop the lives of those she deems immoral for the lives of those she deems worthy. She chose to kill Marshall Hall because he was an openly gay teacher.
"So Marshall Hall died to save me?" Dean asks, just to make the connection absolutely explicit.
The gay teacher’s heart is effectively “given” to Dean.
In the subtext - Dean’s heart is forever a little bit gay.
Picture credit to Supernatural Superwiki http://www.supernaturalwiki.com/index.php?title=Marshall_Hall