"Marriage more shameful than adultery"

[Carlton Bach]

Well, almost IIRC. Under Roman law, a proper marriage (conubium) with its attendant connection between families and consequences for property and inheritance could only be concluded between Roman citzens. This is, of course, the heritage of city-state days. Also, conubium itself recognised several 'grades' in which a woman might either fully pass into the 'manus' (authority) of her husband or remain under that of her father. The law also recognised the unions between Romans and non-cizizens with a degree of protection, but never the full measure accorded to citizen-citizen marriages. Concubinage relationships could also be concluded if a citizen couple decided not to marry, and seems to have been common under circumstances where social barriers made marriage inappropriate. They, too, had some standing under the law. Slaves, of course, were legally regarded as things and could no more marry than a plough or a horse.

Interestingly enough, the epigraphic evidence doesn't seem to recognise these gradations, calling the wives of Roman citizens, the concubines of soldiers and the companions of slaves 'coniunx'