I understand the parallel that you guys are drawing, but there's something that you are overlooking:
All of those groups - blacks, LGT, Aboriginals etc, have a history of being oppressed - and generally by whites.
They are also much smaller (at least in North America), than the white population.
Instead of celebrating whiteness, you can celebrate your German, Polish, Italian, Irish, English, Scottish, Danish etc ancestry.
But to take all those cultures together, and call them white, and celebrate that, seems exclusionary, because whites make up such a large percentage of the North American population.