The difference is...dyke is almost always used as a demeaning slur against lesbians. It's not used to refer to someone as simply a "stupid bitch." The reason it's deemed insulting is becasue you're demeaning them by implying they're homosexual. And, in the way you worded it, it definitely read that you were annoyed at a lesbian in your work. It's pretty much the same as if you said "There's this fucking nigger in my work..." and then in your next post said "Oh, he's not black, I just used it as an insulting term." Would you ever do that? I doubt it. And people weren't really calling you a homophobe, they were pointing out that complaining about racism while using homophobic insults is pretty hypocritical.