Two people “living together” is not marriage, it is fornication.
I have not seen where in the Bible that God explicitly prescribe a particular ceremony or ritual for marriage, other than it being a covenant between a man and a woman.
If a man and woman mutually agree to consider themselves married before the Lord and and present themselves as married publicly, then who are we to say that they are not married in the sight of the Lord?
Now, having established that, to be above reproach and “obey the laws of the land”, then it would be best to publicly contract the marriage (at least obtain a marriage license). You can then quell any questions and present the best witness.
If you truly love someone, care about them, and are committing yourself to them, then why would you not publicly contract the marriage? In this way you can legally protect your mate if something happens to you.
Often at this point, people start giving the following rationalizations:
Rationalization: I want to keep my options open
Response: It is pretty obvious if you feel this way that you are not in a marriage covenant, you are shacking up and if you do “exercise” your options it is adultery.
Rationalization: If we get married we stand to lose disability, social security, welfare, trust fund, alimony, etc
Response: Then you are lying and living a lie. I don’t see in God’s word where He would honor something mixed with lies.