Homophobia was so widespread in the ambient environment for my entire life, so it's not easy to say. The earliest incident that I specifically remember which fits the textbook definition was during a high-school Physics class, were a teacher known to go on about her personal views on anything all the time once, and one day homophobia was on the menu.
The reason it didn't stink as much as other incidents was that a group of kids that recently found out I was gay immediately started challenging her (with very naive arguments, but their heart was in the right place).