Having sexual relations before marriage is a sin, however did you know that the bible does not actually say people can not live together before marriage?
That was started by man and not what God said in the scriptures.
If a man and woman can live under the same roof and administer good self control to not have sex before they marry, then they are not sinning or living in a ungodly way.
The reason people made living together before marriage wrong is because it can lead to giving into temptation easier, and cause a person to stumble. Techniqually the bible does not condemn it though.
Another question must be answered is what is marriage?
It is a union between a man and woman put together by God, and it wasn't tell Moses that a certificate was issued for both marriage and divorce. Before there was no going before a judge or preacher to be pronounced man and wife, and then sign a certificate of proof.
We do now have that certificate by law, and that law does not contradict Gods, therefore in order to be legally married the marriage certificate/license needs to be done......