The same reason why "black spring break" exists. Or Night of Joy, or Grad Nite, or Mensa gatherings, or twin conventions, etc. etc. People like to fraternize with people who are similar to themselves. I can imagine that people who tend to get ostracized more than the rest of us might have an even stronger urge to be surrounded by like-minded people every once in a while.
I've gone during that time, avoided the parks that they were officially attending each day (crowd-avoiding strategy), and it really was no different than any other day except the gay people were a little bit easier to identify due to their t-shirts.
And not saying that the OP is a homophobe, but if anyone reading this is, I'd recommend you stay away from the entertainment CMs (including characters). That's all I'm sayin'.
As for Christians and short people -- someone else already pointed out Night of Joy, and even "little people" have conventions, just not at WDW as far as I know.