By "multiculturalism" I mean the view that large numbers of people from any two cultures, no matter how different, can live under the same government with as much or more harmony than there would be if there were only one culture. It cannot work because sometimes the government must take a side. For example:

1. "Bake the cake, bigot". Since the cake cannot be baked and also not baked, either the baker must be forcibly made unhappy or the gay couple must be forcibly made unhappy. If the baker had lived in a place where homosexuality was simply not tolerated, there would have been no problem. If the gay couple had lived where homosexuality was tolerated, there would have been no problem. Only because all three of them live where there are two parallel cultures that differ on the acceptance of homosexuality is there a problem.

2. France's burqa ban. Same thing. A woman cannot both wear a burqa and not wear a burqa at the same time, so a decision must be made. But someone will end up unhappy either way. This would not be a problem if the Muslims lived among only other Muslims.

3. Losing your head over free speech. Same thing, you can't draw Muhammad and not draw Muhammad at the same time, etc.

Rinse and repeat for year after year, with millions of people, and watch the unhappiness grow. Furthermore, for the goverment to enforce whatever decision it makes, it needs all the people under it to see it as legitimate. But that means it's asking for legitimacy from people who it is screwing over. That can't last forever. It also means that everyone has an incentive to fight and fight dirty for control of the government, the better to safeguard or impose their preferred culture. Who wouldn't want to live in a place where that happens?

You can see how the leftist position arises out of this situation. "If you just tolerate everything, there will never be any conflict!" They'll even make the tolerance compulsory.