WHAT if they held a culture war and no one fired a shot? That's the compelling tale of "Brokeback Mountain." Here is a heavily promoted American movie depicting two men having sex - the precise sex act that was still a crime in some states until the Supreme Court struck down sodomy laws just two and a half years ago - but there is no controversy, no Fox News tar and feathering, no roar from the religious right. "Brokeback Mountain" has instead become the unlikely Oscar favorite, propelled by its bicoastal sweep of critics' awards, by its unexpected dominance of the far less highfalutin Golden Globes and, perhaps most of all, by the lure of a gold rush. Last weekend it opened to the highest per-screen average of any movie this year.