It's not? Just like it's any other form of racism is "ok". Also, you shouldn't say "muslims" as there are several muslims who condone what other muslims do in the name of their religion (read:Islam). What you can do, also what I would personally do, is call all religious fanatics for retards.....since thats what they are. Be it Christianity, Islam or whatever else there is (must be hundreds out there).
I work with representatives from both of those two religions and they aren't killing each other over the fact that they don't agree on certain things. Fanatics always have been, and always will be... Retards.