Hmm, I don't know how this got started, but once it got started, there's a really obvious mechanism for continuing and reinforcing it; if you're gay and you want to meet other gay people, and you've heard there's gay people at the opera, then now you want to join the opera. Also if you're really homophobic and want to avoid gay people, then you'll avoid the opera, which might then make it safer for gay people.
I could randomly start a rumour that gay people really love aikido, and if I was sufficiently successful at getting everyone to believe it, then maybe it'd soon become a self-fulfilling prophecy - since homophobes would pull out of aikido, and gay people would join.
I wonder how you could test this? Could you just survey some opera fans about why they enjoy opera, and see if anything shows up as correlated on the demographic-monitoring bit of the survey? Could you do some kind of experimental design where people are told about an imaginary new hobby you've invented, and told that it's common or uncommon among their demographics, and then they rate how interested they are in the hobby?