Can you tell me why this party is wrong?
Religions are by nature exclusive, and girls generally would be in danger participating in sports activities with boys, so the examples of exclusion that you have cited seem innocuous to me.
On the other hand, everyone goes to the prom. People who are free to attend the prom with their friends will be actively excluded from the prom party to which their same friends will be welcome. Why? Because they picked the ethnicity of their parents poorly.
If you are trying to sow distrust and enmity between races, if you want to have another couple of centuries of Americans hating Americans, if you want the South to continue to be berated for being full of backwards, ignorant, and petty people...then I guess this move is not so wrong after all.
I had hoped we were headed in a much more hopeful and enlightened direction. That's why I think it is wrong. I'm sorry that it is not obvious to you.