"Queer" is a slur and I'm generally against people using slurs. If someone wants to call themself "queer" then I mean, no one's stopping them, but if someone called me that then I would confront them about it. Same thing with "bitch", for example-- some women are just fine using "bitch" casually for themselves and their friends, I am not.

Just my two cents. Slurs like this are degrading in part because they don't just refer to a demographic, they refer to it in a negative and dehumanizing way. E.g. for "queer" it means weird/strange in a negative and other-ing way. And I am generally against dehumanizing language, I think the way we speak often sends implicit messages about our values-- even if we don't intend to spread those particular values.

edit: typo