What about vows? In the UK, at least, it's normal for a wedding to include vows - promises if you will - of how the couple will devote themselves to one another, sacrifice for one another, et cetera.

The institution of marriage has at it's centre the continuation of family whilst co-habitation does not. Marriage is also a lifelong commitment, co-habitation is not.

It's maybe like saying there's no difference between soldiers who've vowed to serve and protect people and if necessary lay down their lives doing so and hired mercenaries who're doing a protection detail.