Why is "gay" an insult?

Oh my gosh! Thank you so much for posting this. This is a big pet peeve of mine. A couple of weeks ago, I snapped on a boy in my Spanish cl for using the word in the wrong context. My teacher even thanked me. It's completely unnecessary and inappropriate. The only way the word can be used is to say someone is or something is happy. If you think a shirt is "gay" you need to rethink because shirts can't have ualities. By the way, I am neither gay or and I have strong feelings about this.