I find that interesting, as Indians in the United States overwhelmingly marry within their own ethnic group.
In India, it gets a bit difficult to define an ethnic group.
Remember, each state in India speaks a different tongue, linked together only by Hindi or English.
So it shouldn't surprise anyone that Indian Tamilians(not an ethnic group, but speakers of the Tamil language) marry Indian Tamils. Or the same with the other thirty states.
These days, even this is changing.