our society includes people of numerous backrounds with different ideas about marriage. some ideas secular, some religious, some have unique perspectives somewhere in-between. People in our society have choices, the law and  religion are separate.  in our society you don't need to marry to have intercourse. Most Christian churches teach that you should be, but it's a forgiving religion so there are no strict penalties if you co-habitate without marriage. Many people think it's ok as long as the couple loves each other. People who haven't been married and had been living together might change their mind about what legal marriage means to them, it might mean a stronger commitment for example, and that may be what makes them change their mind to get married.