It's more complicaated than this. Historically, every cosmopolitan gathering has been multicultural (by definition). "Social cohesion" is more likely in larger cosmopolitan areas than it is in smaller groups. What destroys social cohesion are insular groups that wish to prohibit mixed marriages (eg. white supremacists, orthodox and conservative Jews, orthodox Islam, conservative Hindus, traditionalist Japanese, traditionalist Chinese, etc. etc.