People do marriage to give their children a happy and prosperous future,to give them proper designation and offcourse to give them security.Marriage is like a bond to keep the man and woman together for long time.Sex is not the only thing in marriage.You can do sex without marriage and can have children-just think about the difference,you will understand why people do marriage.
ehmm .. People get marry because they want Childrens so that they can treat there childrens same as there parents did to them Just a thought!!