Living Without Marriage
A formal and acceptable way to declare publically a man and woman to be husband and wife is necessary because this is not just man and woman living together but they are going to establish the unit of a family. Children will be born and the parents are obliged to take care of them. Such a serious act needs pledge of responsibility which is provided with the contract of Nikah.
Also, according to Ghamidi Sahib, a marriage does require a contract or legal registration, but a couple publicly announcing their marriage and calling themselves husband and wife.
Simply these are the boundaries of Allah. We have to obey Allah’s commands.