''Blaine is NOT bi. He is gay, and will always be gay. I think it's very important to young kids that they know this character is one of them."
I guess by "young kids," Murphy means young gay kids and not bisexual ones.
Obviously, I appreciate gay and lesbian characters in the media, but is Murphy unaware that self-identified bisexual characters are hardly ubiquitous in TV and film either and thus might also like to see a character who is "one of them"? His statement implies otherwise.
Even characters who have dated both men and women are often placed into a "gay" or "lesbian" box once they begin dating people of the same-sex, with little insight shared with viewers over how the characters self-identify. I'm thinking of Willow Rosenberg, from Buffy, for instance, who had a loving relationship with Oz prior to her loving relationship with Tara (and Kennedy, ugh barf). As a fictional person, she may very well have been a lesbian, but there was room for her to be written as openly bisexual.
Throughout the show Willow self-identified as "kinda gay" to "gay now," yet she clearly loved Oz when she was with him and indeed told him that some part of her would always be waiting for him to return, even when she's "old and blue-haired." Does writing Willow as a lesbian imply that the Willow/Oz relationship was not authentic? Was she heterosexual when she dated Oz and then transformed into a lesbian once she
Perhaps written with good intentions of depicting Gay Character For Representation Purposes, some characters nonetheless both elide the fluidity of human sexuality and invisibilize bisexual men and women. At the same time, gay producers and writers, like Murphy, might be intent on labeling a character gay out of a sensitivity to homosexuality being used as a plot device to bring in ratings.
You know the story.
A main female character is Fed Up With Men, or briefly considers letting herself be wooed by a hot lesbian, and/or goes to a gay bar with a lesbian (I'd include bisexual woman here, but really, it's usually a lesbian). A kiss between two women happens somewhere in there but by the end of things the main character, surprise surprise, turns out to be heterosexual, and the Guestbian vanishes, perhaps into the same dismal abyss that holds Tara MaClay and other victims of the Dead Lesbian Trope.
Although I'm not sure the kissing gimmick occurs as much with male characters, I could see why a gay writer for Glee might want to make sure one of his characters is Definitely Gay. (Although, I would also argue that Glee's depiction of cheerleaders Brittany and Santana has elements of les/bisploitation).
Yet, in Murphy's clarification of Blaine, I'm picking up on subtext that a character "turning out bisexual" rather than being full-fledged-Kinsey-Scale-6 gay is... a bad thing, somewhat akin to a character going through a gay phase and ending up heterosexual.