When a man and a woman get together, it is not unusual that they would make a baby. Society is interested in marriage to be sure that somebody is taking care of that baby. Otherwise, it becomes our job. Indeed, in most cases where the parents are not married, supporting the kids has become our job, a job that society is not very well equipped to do.
SO, if a man and a woman are going to get together (I am trying to avoid vulgarity here), it is safer for everyone if they get married first.
If they do not, for one reason or another, make a baby, there is no harm done. If they want to be married but don't intend to make babies, or don't even think they can make babies, it is still safer for society and for any surprise children if they get married first. We don't really need to know all the details of what they do together.