In Bruno, one finds confirmation of every negative stereotype of gay people imaginable: Bruno is self-centered, shallow, perverted, sex-crazed, dim, and worst of all, he's completely unrestrained in his sexual urges to the point that he is unabashedly hitting on straight guys. That's one of straight guys' worst fears and Cohen and Charles are trying to attack American men for reacting poorly when a large and imposing gay guy tries to hit on them?
Sacha Baron Cohen's thesis is that Americans are secretly culturally ignorant. How exactly was the tv audience supposed to react to a man bringing an infant son to orgies in a jacuzzi and calling him OJ, or how was the hunter supposed to react to being hit on twice by a guy on a camping trip, or the senator? If a man crossed such sexual boundaries with a woman that Bruno did with Ron Paul or the hunter, it would be no laughing matter. If the point of the film was to raise awareness for homophobia, how did that work, exactly? People weren't uncomfortable around Bruno because he was gay, he was a nuissance. Borat was a nuissance too, but the character himself had good intentions.
Therefore, Bruno's theme "American men are clearly ignorant of gay people and more uncomfortable around it than they care to admit" gets overshadowed by the fact that in the world of Bruno, gay people are just plain undesirable to be around.
There's the battle over proposition 8 and the fear among anti-gays is that the homosexual agenda is invasive and pushing to change America. In reality, gay people just want to be treated equally and be accepted for who they are. I can't imagine having a fame-seeking narcissist who wants to shove his sexuality down other people's throats as the most culturally visible representation of homosexuality at the moment, is what gay people need.
I think images make a big deal too. When I was young, I was fairly homophobic and uncomfortable with the idea of homosexuality until I actually started meeting and befriending gay people in college and realizing that they were pretty normal people like you and me. There's a big battle for public opinion out there and there are a lot of people who are fearful of homosexuality because they haven't had much contact with it. For them, the image that Bruno promotes is something that quells the fire.