For those who do not understand what I do not get......there is no law (at least as far as I know) that obliges people to get married. Marriage registration is for fiscal purposes, to avoid bigamy and to protect children. But the bible nowhere obliges you to get married and have it registered with the authorities. Therefore, in order to be considered married in WT, they (WT) require a formal registration. This is not a scriptural policy but a WT policy. In the scriptures, when a man and a woman slept together, they were considered "married" (belonging to each other).
Moses gave a note of "divorce" in order to allow the woman to belong to another man. in those days, a woman would only leave the parents in order to belong to a man. They would not live on their own and if they did, they were likely considered prostitutes.
If the WT would be consistent in their doctrines to apply old hebrew laws and practices, they should accept a couple (man / woman) that live together as belonging to each other. But as usual, the WT creates its own universe with a mix of WT laws that are either based on certain scriptures, governmental law or on their own ideas. Cherry picking at its best.