Srsly. Like, unless you mean HAPPY, it's not. You cannot act 'gay'. You cannot feel 'gay'. You cannot feel like you have brown hair, or green eyes, unless you're staring at yourself in the mirror. You cannot look at someone and go, "Wow, he's so gay" because you really have no guarantee that he is, whoever he is.
This goes double for non-canon pairings. "Why did you make him gay? He doesn't act gay!" You cannot act gay. Gay is a sexual orientation. You cannot act straight, either. You cannot act black. You cannot act Jewish.
You can act stereotypically gay, sure. But that's not GAY. ALL gay people don't act the same, so GAY cannot be used to describe someone's personality. Please, world, get that into your head.
Sometimes I joke and say things like, "Wow, that scarf is so gay!" I'm just fooling around. I'm talking about people who take things uber seriously and go like, "You can't ship them. You're making him gay. He doesn't act gay!" or "God, why is he acting so gay?"