I had to leave my last job when I realized I would have to go back to the long hours (pumping also - not so fun); there was just no end in sight. Once you set a precedent like that, it's hard to reverse - I admire people who can cut back successfully and reinforce their boundaries.
Now I work freelance - still long hours sometimes, but it makes a difference when you can decide how to organize your own time, plus I work at home.
On the flip side, when my sister was single with no kids, she was always the one asked to work evening events so that the people with kids wouldn't have to. Just because she didn't have kids, doesn't mean that she wanted to do that every time. "Getting ahead" in that way wasn't important to her.
I think the core problem is work-life balance, whether you have kids or not. Unfortunately, a lot of jobs don't respect this. Too bad more jobs can't be part-time.