What a strange complaint. The mouseover text clearly says "What do you want me to do?" - so xkcd author Randall sees it as being in the first person. Arguably assuming it to be a female calling is heteronormative, but then other strips suggest the character has relationships only with women, so. Umm. I say it's a dude. It's also, y'know, a cartoon.
What, people on an academic heavy website getting stuff out of proportion? No, I refuse to even countenance the possibility.