This is sometimes referred to as "assortive mating". In the 1970s, a male executive was more likely to marry his secretary and a male doctor was more likely to marry a nurse, and nobody would think that unusual. Culturally we have shifted away from those norms and nowadays the executive marries another executive or white collar professional and the doctor marries another doctor. This reinforces inequality in many ways (income, culture, parental education and involvement.) There don't seem to be any clear answers for how to address the implications of this trend.