It isn't the whole reason the fandom is like this, but it's always messy when progressive representation is also involved (or claimed to be involved).

It's so joyful when it's going well, and so awful when it's not. It's harder not to take it personally.

Which is, to be very clear, NOT an argument against shows with, say, out and proud bisexual characters! Just for people, in fandoms where they take especial joy out of representation to brace themselves for sourer feelings than otherwise, when something happens that they don't like.