I will ask differently.
Is the blessing of oficial authorities (government, priest, slave master) needed for a marriage to be constituted?
If that's what the governing authorities you are under require, then, 'yes', it is needed.
If not, what is actually needed?
In the absence of legal requirements a commitment of both parties to be faithful to each other is needed.
That's what a marriage certificate does: It expresses your commitment to be faithful and exclusive to one person. People who want to be married without a lawful certificate (if one is required where they live) are actually demonstrating their lack of commitment to the person they are with. Not wanting a marriage certificate on the grounds it's 'not needed' is actually a veiled desire to not be in a commitment to the person they are with. Ultimately, they want to keep their freedom to walk away when times get tough.
And how do concubines in Israel fit into it?
It fit's into men's natural lust for more than one woman--that's where concubines and multiple wives fit in. It was overlooked among the people of God before the time of the indwelling Holy Spirit. But now that all the people of God have the Holy Spirit within them it is wholly unfitting for them to not walk in God's original plan for one man/ one woman for life. This natural desire for more sex than one woman can provide comes from the flesh, not from God. That's why there's no excuse for a Spirit filled person to submit to that carnal desire any longer.
