My definition of mainstream culture is indeed clear: agreement on core issues, not trivial differences. Are you saying that Muslims do not meet that fundamental criterion?

As I said, I have no problem with celebrating and adopting  what is good about different cultures, we have always done that, that is NOT "multiculturalism", it is the melting pot. 

Pizza and spaghetti and Chinese food all became American and Canadian food under the melting pot. Today who has a problem with Thai food, world music, etc.? No one. People trying to pretend that is the issue are beating a dead horse to distract from the core issue. A society based on preventing women from interacting with unrelated men is NOT one of the good things to adopt.
 
The problem is an official "multiculturalism" that encourages the development of parallel cultures based on fundamentally different principles from mainstream culture.