Why do people who don't go to church or believe in religion get married?

I do not know how or why people get married when they are not religious, for it is a holy sacrement, where one vow before God, their family and friends, that they will chrish and love each other until death they do part. How can one vow to someone such as God when they do not believe in God? WHIL I may not agree that they should be allowed to do so, I do not judge them, lest I be judge. The Reason The Goverment needs to know is because you are legally responsible for your spouse and so it is for benefit that you are asked to pay less taxes than that of someone who does not have that legal responsibility. =P