When I heard about this, it annoyed me so much. Some days, I think the Western world is past all the homophobia that goes on, and then something like this crops up and I just lose all hope. Okay well that was a bit dramatic - I have the benefit of living in a country where this kind of thing would never be tolerated (at least, I hope).
The idea that cancelling the prom takes no stance is a very heteronormative one. In this society, it is expected that a boy will go to a prom with a girl. The idea that, if a boy goes to a prom with a boy, or a girl goes to a prom with a girl, the prom should be cancelled in order to keep the school from taking a stance is just... well, it's discriminatory. It's denying these kids the right to do something that it would be absolutely fine for a heterosexual couple to do. It's taken for granted that the heterosexual couple gets to go. I mean likewise, if cancelling the prom because a person wants to go with a partner from their same sex is supporting LGBT rights, then in the sake of fairness, surely the prom should be cancelled if any heterosexual couples go? Otherwise it's just not fair.
Urgh, what I'm trying to say is, doing nothing is as discriminatory as outright banning them. Perhaps even worse, because now this poor couple get all the wrath from their schoolmates for getting their prom cancelled.
And no, being gay isn't exactly the same as being as being African-Americans, or Asians, or any other minority. But in its essentials, it has a lot in common: people being discriminated against for something they have no choice over, whether that be skin colour, sexual orientation, gender or disability (such as autism, aspergers, dyslexia etc).