I think people can have concerts wherever they like but if they have a concert that is open to the public it should be open to everyone.
Stores can be open or closed to the public whenever they see fit. You shouldn't have to provide your services 24 hours a day but when you do provide a service to the public the public should be treated equally.
Again, the KKK is a terrorist organization and you can refuse service if there is reasonable concern for you or your customers personal safety. And a white person can file a discrimination suit against a web designer if they feel like they are being refused a public accommodation because of their race. A homosexual does not have same protections in federal statutes.
And quit saying "participate in". They aren't asking for gay sex. You aren't participating in homosexuality.