Is my school phobic, and wrong for doing this to me?

If it were just you guys wearing the shirts, I would say that the school has an issue with the statement in general, of expressing your relationship on your clothing, it isnt really appropriate school attire whether your gay or straight, but if they have no problem with straight couples wearing them and only have a problem with you, fight the case, bring up the fact that the other couples are allowed to. It could be a misunderstanding, maybe the principal wasnt aware of the other students wearing them and they would have asked them to turn it inside out as well if they knew, but fight the case and get down to the truth.