Cultural Clashes
 
As we become more of a "global community" this is far more common that it used to be. Consider:
the "monocultural" couple that lives in a "local society" that is different (e.g., Muslim couple in an all-white neighborhood)

the "bicultural" couple (Jewish husband, Hispanic wife) who have to build their own "local society." This includes cross-ethnic, cross-faith, and cross-class marriages. Family norms and acceptance (especially parental acceptance), sex-roles, language barriers, ideas about career and money, views on child development and parenting... are all likely to differ.

the CDC study showed mixed-ethnic relationships are more likely to break up (31% same-ethnic versus 41% mixed-ethnic - that's a 33% increase in risk). Finding key common ground may be an important starting point, then work on how they may have forgotten, underestimated, or only superficially explored cultural differences may come next. Alternately, the cultural differences may become "cultural camouflage" that obscures underlying emotional issues that have been ignored for years.