I've been called "gay" before, my friends have been called "gay" before. Heck, even my teachers have had the word shouted at their faces by disrespectful students.
Is it right? No. But is it going to stop? Eh, I doubt it.
What can we do about it? As far as I know, it's something so widespread that it would require a massive effort to fix it. Alas, I rather doubt we could organize something like that. But it'd be fun to think about it. So with that said, I don't think we can really do much about it directly.
Indirectly, however, we can try to influence the people around us. Everything starts somewhere, so why not have it start with us?
If you're able to, please try the following:
Approach at least two people that you know who has used "gay" as an insult before. Try to discuss the topic with them, and perhaps show them some of the posts that people have made here. Explain to them how absurd, harmful, and toxic it is. If possible, try to convince them to watch their language. It's possible to use clean language, but still be "cool" or whatever it is that they try to do by talking like that.