A lot of the replies are close, but do not accurately answer the question. The reason people always ask that question is because it provides an easily accessible conversation topic, especially in circumstances where two people know nothing else about each other. Everyone can relate to marriage in one way or another (regardless of being in support of or against that social institution).
If you were married the default topic would be, "When are you having kids?" or "How many kids do you have?" or "You have kids? What are their names/ages/etc.?" Again, people are just after an easily accessible conversation topic -- people feel the need to fill silence or break ice with these topics which are universal regardless of your background.