To give you the benefit of the doubt, I think the reason that gay characters often feel so hamhandedly introduced is that it's not immediately obvious if someone is gay or not (unless you're a rainbow-haired short-shorts-wearing rollerblader), so the gay character has to announce their gayness for people to "get it." A black character doesn't have to tell you he's black; it's immediately visually obvious. In the zeal to make sure people understand the gay character's gayness, the writer may make the character's sexuality a large part of their personality, interests, etc. which can quickly become grating. Other characters can have various, disparate interests, but not the gay character, he's just gay. Since homosexuality is a sexuality, you're also bringing sexuality into stories where sexuality is either nonexistent or kept to a very minimal level ("romances" that mostly hinge on characters liking each other and maybe sharing a single kiss at the end of the day). This can be awkward.
Ideally, for kids shows, a character being gay is something that shouldn't be immediately, glaringly pronounced. People get mad when characters are only gay in the background, but I think it's better than having the character be solely defined by their sexual interest in the same sex. It conveys that gay people are fully fleshed out and realized human beings defined by more than just their sexuality, which seems a more positive and inclusive message to me. If the most interesting thing about a character is who they want to (suck and) fuck, then they're not an interesting character.
You have more latitude in material not aimed at children because you can present more sexual content. Make everyone gay, why not. As an aside, it feels like a lot of current writers insert "adult" themes and topics in kids shows because they want to make important and meaningful stories but they don't want to struggle with the complexity and nuance expected of actual adult entertainment.