You're conflating "social class" with "economic class". The old marriage thing was more about social class than economic - a man could marry a girl with no money if she came from a "good" family but marrying some rich girl with no lineage would be frowned on. (see: All of Downton Abbey) Also, one could be from "respectable society" but not be rich, if you had "good blood" or "good school", etc.
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.