He didn't say 'He's in the gay play, he MUST be gay.' He's asking a simple question.
A questions that is neither offensive nor, as far as I'm concerned, too personal (though many others will disagree).
Perhaps this guy has a crush and wants to meet him and maybe ask him out for a drink.
Maybe he's a young gay guy who is struggling with his sexuality and would like to feel less alone by knowing an actor whom he admires is gay...like him.
Or maybe this person really does believe that to play gay you must be gay.
But to call this person an idiot for asking the simple question is a bit harsh.
"I'm so looking forward to a time when all the Reagan Democrats are dead."