Marriage - What is it?

Every society is an ensemble of families. Families come together and define a society. This is the definition I have learnt and seen myself in India and now in the US. This entity called family is formed due to a concept called Marriage. This has been a real confusing concept for me.

I'm no person to question the meaning of the institution called marriage. But what befuddles me is what binds a man and a woman together for life. Right from our birth we use resources of mother earth. We use and refuse. We buy new clothes and use them till they either wear out or we feel that they are not worth being used. We buy cars, computers and a whole lot of items manufactured by businesses. Sooner or later we junk everything we buy.

But when a person marries he/she thinks that the partner is one with whom the rest of our life should be spent. But why does the normal human mind thinks so? (There are many exceptions to this but atleast 70% of marriages in the world last for a lifetime). What is the binding factor??

The physical aspect in marriage should play a very big role atleast in the initial days. The physical needs of a man and a woman need to be satisfied and marriage should be providing a means for that. Is sex the binding factor?

Sex begets the act of procreation. So sooner or later most couples get kids. Is that the prize for being together? Or is it the factor to bind the couple for the future?? A normal father and mother would want to bring up the child and set an example for the society and in the process stay together.
Is the child the binding factor??

Many a time man and woman without children stay together for a whole life? I'm confused. It befuddles me. May be sometime when I marry I'll understand what it is !!!!