Hrm. Kind of a context thing. Somewhat like the word "girl" when used for a woman - a grown woman might say, wryly, "I'm a big girl, I can handle it," and that's fine, but if a man refers to her dismissively as "some girl" that's sexist. Similarly, my best friend will jokingly refer to me as "a big old lesbo," which is fine, but Batman using the word to categorize and dismiss a woman with whom he has no relationship is homophobic. Very mild, yes, bottom of the list of linguistic priorities. Not something I'd ever typically call out, personally. But still a slur - the innocuous use actually requires it to be, because the joke is playing on the expectations already attached to the word (as with the "big girl" example).