I'm a third-year (male) med student who unexpectedly loved my ob/gyn rotation, and am struggling a bit with the same issues/questions. (It's not just women who want to have time for a family and a life outside of medicine).
Ob/Gyn is simply never going to a be a "lifestyle" specialty the way that a field like derm is. But I have the sense that just how bad the lifestyle of a practicing ob is depends a lot on the individual and the practice they belong to. I've met some docs who say it's a very demanding field, and others who say it's changing and that there is a way to find work/life balance, so I don't know if there's a straightforward answer to your question. There does seem to be a trend, at least where I live, of Ob's practicing in very large groups so that they can share call; the call nights are still pretty rough, but for an attending in this sort of set-up they might be on call only once every 2-3 weeks. If anybody else cares to comment, I would love to hear your thoughts.
Fortunately for you, you have a few years before you have to make any hard decisions. Seek out some mentors in Ob and talk to them about what their life is like. You'll also have plenty of chance to evaluate the field first-hand when you do your third year clerkship. Most importantly, keep an open mind (about Ob and all fields) and take what you hear from people who aren't directly involved in the field with a grain of salt.