Most of the posts written so far seem to be along the lines of "Startups, marriage and children can all be made to work together." Patio11 makes a good point about the truism 'Startups take over your life' being a somewhat artificial construction.
I will play contrarian and argue the opposite side of it.
No matter how well some people manage to integrate their work life with their family life, they would still be more free to compete if they had no family.
Maybe you have created enterprise software and a large company in Texas, USA, calls you and asks to come pitch to the CEO tomorrow. How free are you to get out of any personal obligations so you can be there in Texas tomorrow?
Maybe you are selling support for open source software, are you free to work 80 hours a week for the first 2 years, just to get things going?
Maybe you are focused on electronics in Asia and you are trying to learn about the various laws and electrical infrastructure requirements in each country. It would also be nice to know something about the culture of these countries, their customs and history - each week, do you put an extra 5 hours toward learning background information about these countries, or do you put that extra 5 hours toward your personal relationship?
My overall feeling is that you can start a personal relationship at any point between the age of 14 and 84, but the first 2 years you are trying to get a startup off the ground is unique, and I think during those 2 years, much of the rest of your life should be put on hold.
If you end up competing against my startup, and I'm putting in 10% more than time than you, then you better be 10% smarter than me to balance things out. In my case, that is not very difficult, I am fairly stupid so you might well be 10% smarter than me, but you might eventually face competition that is, in fact, smarter than you, and willing to put in a lot more time than you. What then?
At the risk of sounding like some kind of Zen guru, I do think you need to ask yourself what you truly want. While we are all subject to random chance, there is a large extent to which we can control our lives, and to that extent you will create the life that you strive for. What you think you want may be different from what you actually want, and you'd do well to think carefully about any discrepancy between your stated goals and your actual actions.
Possibly one way to figure out what you would be comfortable with would be to think about losing. If you end up competing in a particular niche, against someone just as smart as you, and your competitor wins because they work 100 hours a week, whereas you decide that your personal relationship, and mental health, are important, so you only work 40 hours a week, then will you be okay with losing? Will you go on to have a happy life, or will you look back later and have regrets? In the book The 7 Habits Of Highly Effective People the author Stephen Covey suggests "Begin with the end in mind." That's applicable here. Think about what outcomes you'd be comfortable with, and let that shape your actions.
Since I'm playing the contrarian here, I will try to end on a negative note: you will have more chance of success the less energy you invest in your relationship. You might be much happier if you focus on your relationship, but we are not talking about happiness here, only the success of your startup. Being able to devote 100% of your energy to your startup might only give you a very slight edge compared to other startup entrepreneurs, but every little bit of an edge helps.