I agree it is class-based, and it's not just about careers.
I think middle class girls are more likely to be told they can do / be anything they want, and so are less likely to rush into a state of affairs that is, essentially, the rest of their lives.
You have all the time in the world for domesticity - the rest of your life, in fact.
But you only have your care-free 20s once.
Which means, traveling, having fun, going out, meeting people, making friends, having experiences, and yes, having a career.
All things most men take for granted. You don't see many teenage boys/men and young 20-something men rushing to have babies. There's too many other things to be doing first.