Any ideas when this became the norm? It used to be seen as scandalous in both directions: upper-class men and women were not supposed to marry into the working or lower classes.
In modern life and in the US, we're much less aware of social class at all (or actively dislike the concept), and it's gotten (somewhat) decoupled from economic class. Someone who has become a lawyer or otherwise economically successful could be from any class and it's not unlikely they might choose to marry someone from a similar social class (with similar background) rather than someone with similar economic class.