First of all, morality is relative. Depending on what the world and the nation thinks, different things become acceptable. For instance- at the time of Rosa Parks' bus incident, the vast majority of the nation and the world was against segregation, in terms of popular opinion. Today, there are only 9 nations in the WORLD who perform gay marriage, including the USA and Mexico who, as you know, aren't very open to gay marriage at all.
The nation's opinion of gay marriage is obviously that it is immoral. Almost every time a state or locality has allowed gay marriage, it has been forced back out.
If gay marriage is immoral, why should it be allowed in our public schools? We don't allow the kids public displays of affection in school for the same reason.
Second reason racist discrimination is different: your race is clearly visible just from looking at you. It is something that you can't hide. If these girls really wanted to go to prom together, they could have found a way without official involvement. They could hide it. Instead, they decided to rub it in everyone's face by one of them insisting on going so far as to wear a tuxedo. I wouldn't want them letting in a person wearing assless chaps, either- it would just be an immoral, distracting, selfish act of defiance against the norm, ruining everyone's magical night.
And yes, I already said that the school was wrong to allow them to ruin prom by just shutting it all down.
The discussion wasn't (therefore the question) morality, but discrimination. That schoolboard, Rev. Fred Phelps, Catholic Priests, etc. do not own the copyright on morality. Morality is an opinion, discrimination is a crime. Please stay with the conversation.
Just because you can "tell" someone is black is about as laughable an argument as I've ever heard. Can you please explain what that has to do with the argument as well?
I also categorically dismiss your points on "if the majority is against it, it must be bad" because the majority of the country (including Lincoln) were NOT against slavery continuing to exist in states where it was already legal. Your argument suggests that slavery is okay under that standard. It suggests also, that because the National Guard had to be called in to enforce Brown v. Board of Education means that was the wrong decision.
Finally, as far as "hiding it". Why do they have to? If PDA is forbidden (and have u been on a high school campus lately?) then it shouldn't matter either way.
Since I have your attention, maybe you would take a moment and explain your orientation status of 70% straight and 30% gay. Is it another case of latent homosexuality where your hunger for the same sex has to be eclipsed and hidden from your wife, kids, pastor, neighbors, etc.
It seems to me that is what you are advocating. Be gay, just don't tell anyone? Which leads to seedy bathroom sex, the spread of diseases, lies, deceit, mistrust. Shouting from the mountaintop that homosexuality is immoral while constantly on the lookout for the next big dick to suck is a double standard.