My SO and I have a 9 year old, been living together for 7 years. My son is of the age that he knows what being married is and that most of his friends parents are married. By the same token, he knows that we aren't, and that presents a problem at times when someone asks why we haven't married in front of him.
To protect him from the things that could be uncomfortable for him, we've decided to get married. The stability factor for him, not having parents with different last names, etc. And, it's the right thing to do.
We'll be getting married in Maui on 7/7/07, come join the party!