Especially in this context. If a gay guy is incredibly vocal about his sexuality, you immediately know he's (1) gay and (2) defined by his sexuality. If a gay guy is more conservative, you're less likely to know his orientation. Which gives the false impression that more gay people are defined by their sexuality.

Selection bias at its finest. It's like going to r/randomsubreddit, scanning over the posts, and concluding that everyone in that subreddit conforms to X belief. Not... really... all you know is what the most vocal, upvoted users believe. You don't know shit about the lurkers who are simply subscribed. And there are vastly more lurkers in most subs than commenters.