Andrew Kahn writes at Slate:
The trope is deployed not because it is novel but because it is not. It is hammered like a schoolyard taunt, with a smug assurance that Trump has been duly “trolled.” Like a lot of liberal comedy right now, it serves a fantasy of resistance through snark. (Think of Seth Rogen’s dumb Twitter messages to Donald Jr., recently glorified on Colbert’s show, or the news—ecstatically received in February and then chronicled as legend only three months later—that Melissa McCarthy had humiliated Sean Spicer.) Sexual politics aside, our glee in calling Trump gay says more about us than it does about him.
As for the sexual politics: The joke is a caricature of homoeroticism, descended from century-old stereotypes about gay men and power. Yes, most of the participants are angry liberals, idly imagining how cool it would be to make Trump sad—but Trump is not in the audience (most of the time), and he doesn’t seem to care a whole lot about homosexuality anyway. So why is this particular image the weapon of choice?