I think it's possible to have a startup that promotes a sustainable lifestyle, but it's all about costs and benefits. If you work 100+ hours per week, you might miss out on seeing your kids grow up (or having any in the first place). Working ~50 hours instead might make your business grow more slowly but might allow you to enjoy other (more important, for some) aspects of life.
As a side note: some of the investors I've talked to also encourage and prefer a more sustainable lifestyle in a startup. That way you'll be alive in the long run. :)
I don't want to name any names or give explicit examples here, but happy to discuss you're making a difficult decision.
One of the startups I founded required 10 hours a week from day one. Made my first million. That's when I realized that the number of hours you put in to something doesn't necessarily reflect what you will get back from it.
Another startup I founded required 60 hours a week in the beginning, and 5 hours a week down the road.
These days? RescueTime says I'm at 61 hours this week so far. Obviously not sustainable, but it's what needs to be done to reach my goals. I know where I want my startup to be, and what it will take to get there. It's important to be honest with yourself.
I keep my significant other in the loop so that she understands what I'm doing, why I'm doing it, and why I'm putting in long hours. She knows that right now I'm working hard, but there will come a time when I won't have to put in long hours. Communication is key.
There's something to say for efficiency too: make better use of the time you do have. My personal efficiency hovers around 92% — While I'm at the computer, I'm unproductive only 8% of the time. If you're working 40 hours a week and you improve your efficiency by 25%, now you can do 40 hours of work in 30.
I've been terrible at that in the past. Any tips?