I can see grounds for considering it arson, and/or assault, if the flag was set fire to in a way that it could cause another structure to also burn (while being wary of prosecutorial manufacturing of mountains from molehills). Also, if it was burned in effigy, perhaps draped as a cloak over a likeness of a particular person or stereotype of a type of people.
If text accompanied that said "all gays deserve burning" or similar while being done in front of a gay support group or deliberately gay function, I could see it being interpreted as assault even without arson.
Burning the flag alone, on a driveway or similar noncombustible surface, and without theft, it's just an act of speech. To me at least. It reads more as a strike against special interest politics, than an act of aggression.