See this is the problem. As soon as homosexuality is mentioned 'it's none of your business. Stop talking about it. Who cares!!!"
....and so on.
To be fair there are plenty of threads speculating as to known heterosexual people's private lives, work lives, financial lives and some posters write "its none of your business...". Its not exclusive to gay themed threads. But as this is a gay themed thread it is only natural someone will write "its none of your business." That's not to say it is taboo, just the poster's personal preference not to speculate as to someone's private affairs.
Not to mention the number of threads on DS about possible gay celebrities would suggest few think it is a taboo subject.
As a gay man, I might sometimes speculate as to a person's sexuality, but I don't take offense if another person doesn't want to. I'll save my frustration for out and out bigots.