Yeah, it's homophobic because it's not simply a sex joke, and it's not a play on their hypocrisy. There's no other comedic potential left except, "lol he's gay." That said, I'm not particularly offended by it because it's a spray paint job, there's only so many opportunities available in the dark of night, and I don't really hold vandals to such high standards as to expect them to not be problematic. Getting mad about this is sort of like being disappointed that the number written in the men's room stall did not, in fact, lead to a good time.