Well this is a classic example of an overreaching blanket statement.
Sure, you may be bullied because of your sexual orientation, but PLENTY of homosexuals are bullies themselves.
I say below why the word 'gay' is used as an insult; it's because that word originally meant 'exceptionally happy' and nowadays can be used to describe anyone who is very eccentric, not necessarily a homosexual.
There's a lot more nuance here than just chalking up all bullying to homophobia.