While I agree birth control facilitated this, I think the women's movement was also the force behind changing women's expectations about what was possible and what they should have the right to do. It seems among younger women, "women's lib" and being a "feminist" have become negatives. But it was the first brave women who fought for the right to be in law school, and to get hired at a law firm as a lawyer - not a secretary - that opened the doors for all the women who take those rights for granted. And they put up with lots of harassment and inappropriate treatment to survive - plus often had to work twice as hard to prove themselves, paving the way for the next generation's easier path into those jobs.
I remember my first job interview after college in the early 70s. The questions weren't about my skills and experience, but about was I married and what was my husband's job - and they weren't being asked just to be social but to decide if I would want to stay in the position for more than a short period!