If you meet few lower class people in your daily life then you are not likely to marry one. When it comes time to marry you are likely to be living and working in an environment that is homogeneous with respect to class and race - but not much else. It is likely that religion and ethnicity will lose importance as endogamous norms. There is any evidence that social class has declined in importance in the mate selection process. It was important for your grandparents, your parents, and will be for you too.
Most people end up marrying someone from a similar social class background.
Where there is a difference in social class background, then it is usual for the female to be f a lower social class than the male. Exchange theory has a good, easy explanation for all this. Class affects how we are brought up, how we speak, our values, our expectations, as well as our life experiences. We come into contact with them much more frequently than with others. We tend to marry them.