I don't think I want to attend a friend's gender identity reveal party.
It can be fun! Why don't you want to join in on the fun?
I don't care about gender, and I thought it would be amusing to ignore my friend.
It's fine to not want to attend a party if you don't want to. I don't think you should ignore them, though. Just let them know that you won't be attending.
Screw him. He is stupid and he can have fun with his silly gender party without me.
I don't think it's right to call your friend stupid, but it's completely fine to not attend the party. Different people have different ideas about things like this, so you shouldn't feel bad about not attending.