Graduate school has a reputation for being a tremendous time-sink. I'm currently married with kids, and I have a fairly busy extracurricular schedule. Is it common for married people - or anyone, actually, with significant external time constraints - to not finish their graduate work, specifically due to conflicting obligations?
I've seen it work both ways: some people realize that they have a limited amount of time to work as a result of their external commitments, and therefore make themselves super-productive during the hours they are able to do research. I think, to some extent, that most of the people with severe external obligations fall into this class.
For a sizable minority, however, the balancing act proves too difficult—although this is often a function of a mismatch in expectations between the advisor and the graduate student. If you believe this could be a problem for you, you should definitely talk with your advisor; if the outcome is unsatisfactory, you should also consider speaking with your thesis committee and the "graduate officer" of your department.
In general, I think that if you are a productive graduate student, your advisor would be willing to work out suitable accommodations for your schedule.