Chris Colfer’s character Kurt (the gay one) was added only after the producers auditioned Colfer and decided to add a role for him. There was originally no plan for a gay character. I see the theme of the show as the struggle of high-school misfits. It’s popular not because it pushes a liberal agenda but because it engages the audience’s willingness to root for the underdog and to remember their own awkward moments of adolescence. It is also, as the OP says, very, very sweet. This post reminded me that same-sex affection is what *really* has Mormons torqued. Kurt is about the only character on Glee who hasn’t had sexual experiences of any kind. The only thing remotely sexual is a schoolgirl crush from afar he’s developed on another character. Contrast that to the amount of sex that almost all of the other characters have had. Clearly, the objection in Mormons to all things gay runs deeper than just issues with the Law of Chastity. It’s gay people–their lives, their affections and their struggle for acceptance– that are seen as objectionable and unwholesome. It doesn’t matter that Kurt is chaste; he’s unwholesome more or less by definition.
(This was posted before Klaine officially became a couple, but I thought it was well-stated.)