Once again I quote mousethief's mum, and apply it to the example troll came up with.

Is it kind?
is it true?
is it necessary?

And you need two out of three.

I'd give that "Straight Pride" T-shirt one out of three. If I'm wrong, and in the circumstances of the school two out of three would be justified, fair enough to wear it. Otherwise not.

"Gay Pride" on the other hand would probably be at least two out of three. Circumstances alter cases.