It's the intent behind the word. The definitions are largely defined by society and contemporary culture. Before, "gay" was used to define something as "happy" or "joyous", now obviously it means something different to people.

Likewise, "Jackass" and "Bitch" had different meanings (and at least dog shows continue to use the latter's alternate meaning).

I don't think homophobic is a slur. I guess it could be used as one, taking into consideration the intent of the user.