I guess if the marriage is within the same cultural/ethic group then the married couple, as well as their families, can better relate to one another in terms of customs and traditions. That is, if they're religious. It'd make it easier for the two families to integrate and accept each other.
However, I think in today's society, there are less people caught up in cultural traditions and all, especially in western societies. Nowadays I see more and more married couples of mixed ethnicity with completely different cultural backgrounds.
I think with today's technology, with the television and novels and magazines/newspapers so widespread, it's a lot easier for people to become familiar with other cultures and religions, and to become more acceptive of another's traditions. I'm pretty sure that a hundred years ago, people weren't as savvy and understanding as today. Other cultures and religions would have been alien to them, so rejection was almost a given.