How do children understand marriage?

A question I've been pondering to myself today is: How do children learn and understand the concept of marriage, especially if their parent(s) aren't married?  

In my daily consumption of pop culture, I've already come across two examples of male celebrities who are openly thinking about getting married due to their (out-of-wedlock) kids asking them why they aren't married to their mother.

1. P. Diddy in his song, "Coming Home":

http://www.youtube.com/watch?v=k-ImCpNqbJw

Between 1:05 and 1:22, P. Diddy raps:

What am I supposed to do when the club lights come on /
It's easy to be Puff, but it's harder to be Sean /
What if the twins ask why I ain't marry their mom /
How do I respond?  /
What if my son stares with a face like my own /
And says he wants to be like me when he's grown?

2.  An article on people.com (<sigh> yes, I do glance at it occasionally...okay fine, daily) where Brad Pitt is re-considering his long-held view that he, "will consider tying the knot when everyone else in the country who wants to be married is legally able."  What's making him reconsider?  His kids asking about it (full interview here).

Preparing to have a child myself, I've been thinking more and more about how I want to teach my kids and how I want them to see me.  Surely, a great deal of their understanding of marriage will come from the model they learn from me and my wife, but in the two situations above, the children seem to know either that if their parents love each other or if they are their parents' children, their parents should be married (or both). 

Is that something learned in school or from societal clues around them?  Or is there an additional instinctive, inherent, natural order to it as well?  At the very least, the family unit is showing to be quite influential to even the more world-hardened people.