Because people used to the word 'gay' as an insult, to insinuate the other person was homosexual.
It gradually shifted in its meaning as an insult, to mean 'stupid or lame'.
If you can't see why that's offensive, then you are stupid. It's like calling someone a jew because they're stingy with money. Oh, it's not offensive because I'm not being anti-Semitic, just making fun of misers! No, shut up, you're stupid.
What the thread is about is streamers saying stuff like "Aww, gay, he stole my red".