I don't want to alarm you, but a lot of the stuff people go on about when it comes to a discussion of work/life balance is about as real as unicorns. The only "balance" is in choosing which ball you are going to let drop today and deciding not to drop the same ball repeatedly. If you want to call that balance, feel free to amuse yourself.
But to me, the key is that last part: don't keep dropping the same ball. There will be days you miss important things with your kid, or push too much responsibility to your significant other. If those days build up you are looking for trouble. There also have to be days where you say "I can't make that meeting" or "I can't be in the field that long" in order to go home early.
You need to make decisions that will allow you to live your life and accept that you won't be making everyone happy all the time. This includes yourself.
If you choose yourself and your job all the time, you'll probably end up dispensing terrible advice on listserves to junior people, much like Clara Jones has done on the Ecolog-L over the last few weeks.
Dr. Jones started off by basically telling all young female scientists that: A) Babies attract bears*, B) Women go to too great of lengths to achieve their goals, C) Science and families do not mix, D) Guilt caused by having a family you can't interact with will hurt your research output, E) You ladies should be the type of women who can lose a kid and show up (on time!) to work the next day, and (my favorite), F) If you have to have kids, consider surrendering custody, like she did!
BUT WAIT! There's other ways to make a family compatible with a career in science, just freeze your eggs!
Seriously, what the hell is with all you ladies wanting to have kids before you are full professor! While we are at it, I propose to freeze one's parents so they don't get old and need help while you are on the tenure track. Probably safest to freeze any grandparents, good friends and pets while we are at it. Oh, and obviously you're gonna want to freeze your significant other** so they don't feel like you're never around.
Which one of you ladies needs a mentor? I bet we can hook you up with Dr. Jones!
If we continue to frame the discussion regarding work/life balance and gender equality in science as "How can you manage your life to be least obtrusive to your career?" then we are going to keep losing good people, and especially women, from science.
*Between babies and menstruation, I don't know how any women goes in the woods without getting mauled!
**If they are also in academics, make sure to freeze them before they get you! Being frozen for a decade or two totally kills your h-index.