I don't think it's tacky if it's not used in a malicious way. Yes there is an element of speculation like with James Dean but there is also well documented gay lives like Porter or Albee.
I think it's tacky if done in a catty way. Or done in a weird way to increase the world's gay population. I remember once in college a guy asked me if I was bilingual. But he put a pause between bi and lingual. He probably just wanted to ask me if I was bi, but because I gave him a strange look, he added lingual instead of sexual. He thought he was being clever, but it seemed like a trap, a way for him to include me in his group of people who are gay or swing that way sometimes. My personal proclivities were really none of his business. And I feel this way about movie stars. What they do in their personal lives is not our business unless it directly affects us. The speculation might stem from curiosity, but often it's a trap to ensnare others, because someone wants to make it seem like everyone is gay, and of course, we know not everyone is.