How will widespread colonization of other planets affect human societies in terms of diversity and tolerance of other cultures?

I'm new-ish to these futurology considerations, myself.

I see it all the time in spec fiction: We on Earth live amongst hundreds of other cultures, and learn (mostly) to tolerate one another. We have nowhere else to go. Colonized planets in spec fiction often tend to have one, or few (whatever suits the story) culture apiece.

If it becomes extremely easy for humans to colonize other planets and our available living space grows exponentially as a result... will we have a tendency towards less diverse, more insular societies? If it's easy to simply go be with your "own people", why would we choose to hang about with others different from us?

Also, what would be the effect over time? To put it in the vulgar, we all know that circle-jerks feed themselves, so I wonder if a tendency towards insularity would foster a decrease in tolerance for others.