Of course, I hope (and suspect) he is keeping his private life private because he wants to. But then, owing to his ridiculously compact schedule, I'm not convinced he has a private life at the moment. Infuriatingly, he might face just as keen a media gaze in his time off, when he's supposed to be, well, 'having fun'. That's when the invasiveness will probably begin to grate. And at some point there will inevitably be tabloid stories of some sort—he's far too handsome to go single for long, right?
So yeah, it's a question we're stuck with, and it will be interesting to see how Mika deals with it as various allegations crop up. The professional response would the one he's giving at the moment ('My private life is private', etc.) But he will do well to keep people curious about his orientation for ten years. Most people will make their minds up quickly if they haven't done already.* On the other hand, even if Mika were to say he was straight/gay/whatever, there's still media attention to be had in who celebs are f###ing. He's in a lose-lose situation.
IngievV, I've been trying to avoid news on Brad and Angelina's relationship for as long as I can remember because it was so drawn out and mind-numbing. But you've saved me there from trawling through at least six months of gossip columns. Thanks. ;-)
* and I should say this speculation is partly fuelled by Mika himself in various songs.