Well, when I first scanned this thread I really had doubts about whether I supported this. Same-sex couples going to proms together yes, but traditionally dressed as 2 boys or 2 girls. In my mind, one of them dressing as the opposite sex reinforces false notions by straights of how our gay & lesbian relationships work.
But then I read the link (I tend to do things backwards; I usually read magazines from back to front, for instance), and my attitude changed. This student is a cross-dresser, probably all his nice evening wear is already strictly female. And I guess his date will be a female (not sure how she'll dress), so it's not like they're attending as a mock straight couple.
Or maybe those considerations don't matter, anyway. Sometimes for all my liberality I catch myself with 1 foot still in the past, and it takes me a few minutes to catch up with the rest of the GLBT community.