I don’t have to broadcast to people I’m heterosexual so why should someone who is gay announce their sexuality?
They don’t have to of course. But if they don’t, people tend to assume they’re straight. So the rationale’s pretty obvious.
Contrast; my brother works in the theatre, and is a wee bit camp, so he finds himself coming in to people, because everyone assumes he’s gay. (this really annoys his girlfriend, people assume she’s his cover.)
But there’s sometimes more to it. A friend of mine was sort of quietly gay, when I first knew her. It wasn’t a secret, she just doesn’t make it obvious. Most people found out eventually (yes, I found out because I hit on her, now she says Pink Triangle is our song, apparently I am oblivious to really obvious clues). Anyway. Every so often, someone was offended- why didn’t you tell me, don’t you trust me? I’m your friend! Did you really think I wouldn’t have your back? And rarer, someone’s a total **** about it- all this time you’ve been lying to me! You’re not who I thought you were!
So now she tends to tell people pretty quick, because at least that way if it turns someone off, it happens fast, and they were quite likely an arse anyway. Makes total sense to me anyway.