1. Bruno is the most OTT character Sacha Baron-Cohen has ever done. No-one is actually REALLY like him. He's just a cartoon character played for comic effect. Just like Will Ferrell did with Ron Burgundy and no-one said that was offensive to newsreaders.*
2. Yes you laugh AT Bruno for the first 10 minutes and then you laugh WITH him - well, the guy behind the mask, Mr Baron-Cohen - as the real targets are revealed...
3. ...the fame-obsessed people of America, from Madonna and her orphans to pushy parents. All actual people, not fictional characters. Bruno wants to be famous and holds a mirror up to that celebrity world. We're watching the reflection.
4. And not forgetting the bigots, the zealots and those who just take themselves waaaay too seriously - we have a laugh at them too.
5. So the fact that Bruno is gay isn't really the main point (it's A point, but not the biggest one). Bruno is shallow and it's shallowness that the film is really about. And then, just like Ali G and Borat do, he realises that love is much more important than anything else. Bless.
6. So that's why. Thanks.
* A newsreading legend told me this. All hail Alistair Stewart.