Then say that rather than "it's so common" because if we were saying that labels that are common don't deserve to be labels then heterosexual shouldn't be a label because it's common to the point that people tend to assume a person is heterosexual until told otherwise.

Yeah, I agree demisexuality is overused as a label, but the fact that it is common doesn't mean it doesn't exist. It just is overused and people who think they are oppressed for being demisexual are morons since it is a societal expectation, especially for women.