Live in had some technical disadvantages.
1) Though it well determine the mother of a person but silent about the father. In a developed and complex society the father also have to be determined for the various practical purposes.
such as who will feed this child; who will be the hair of a decadesd - for the purpose of distribution of the properties left by the deceased, etc.
2) In the meantime society turns from matriarchal to patriarchal society. Property, capital, source of income etc captured by men, and woman lost their seperate financial identity, therefore determination of father was essential and extreme necessary to the society.
To determine the father of a person, the ancient society adopted a theory, which is called ‘access theory’. The access theory, allows a woman accessible by a man only. Here the society introduced the system of marriage.
Firstly it was group marriage, where among a group, all the group members of the opposite s*x was accessible to all. This system also fails to ‘determine the father’ exactly. Thereafter society introduced the access theory, which allows a woman accessible by a particular or very specific,- a man only (Through marriage).
This is modern marriage. Actually it is a ‘paternity determination test’
Now modern sciences explore the DNA test. These tests scientifically determine the paternity of a person. This system, I hope, will replace the ancient paternity determination, hence the marriage. Live in, will get past social support, again. It’s past draw back now overcome by the DNA test.