In Germania, Tacitus describes the marriage and family of the Germanic peoples who caused the Roman Empire so much trouble, and eventually sacked it, setting themselves up as the barbarian lords of its remnants:
Yet the laws of matrimony are severely observed there; nor in the whole of their manners is aught more praiseworthy than this: for they are almost the only Barbarians contented with one wife, excepting a very few amongst them; men of dignity who marry divers wives, from no wantonness or lubricity, but courted for the lustre of their family into many alliances.
To the husband, the wife tenders no dowry; but the husband, to the wife. The parents and relations attend and declare their approbation of the presents, not presents adapted to feminine pomp and delicacy, nor such as serve to deck the new married woman; but oxen and horse accoutred, and a shield, with a javelin and sword. By virtue of these gifts, she is espoused. She too on her part brings her husband some arms. This they esteem the highest tie, these the holy mysteries, and matrimonial Gods. That the woman may not suppose herself free from the considerations of fortitude and fighting, or exempt from the casualties of war, the very first solemnities of her wedding serve to warn her, that she comes to her husband as a partner in his hazards and fatigues, that she is to suffer alike with him, to adventure alike, during peace or during war. This the oxen joined in the same yoke plainly indicate, this the horse ready equipped, this the present of arms. 'Tis thus she must be content to live, thus to resign life. The arms which she then receives she must preserve inviolate, and to her sons restore the same, as presents worthy of them, such as their wives may again receive, and still resign to her grandchildren.
They therefore live in a state of chastity well secured; corrupted by no seducing shows and public diversions, by no irritations from banqueting. Of learning and of any secret intercourse by letters, they are all equally ignorant, men and women. Amongst a people so numerous, adultery is exceeding rare; a crime instantly punished, and the punishment left to be inflicted by the husband. He, having cut off her hair, expells her from his house naked, in presence of her kindred, and pursues her with stripes throughout the village. For, to a woman who has prostituted her person, no pardon is ever granted. However beautiful she be, however young, however abounding in wealth, a husband she can never find. In truth, nobody turns vices into mirth there, nor is the practice of corrupting and of yielding to corruption, called the custom of the Age. Better still do those communities, in which none but virgins marry, and where to a single marriage all their views and inclinations are at once confined. Thus, as they have but one body and one life, they take but one husband, that beyond him they may have no thought, no further wishes, nor love him only as their husband but as their marriage.6 To restrain generation and the increase of children, is esteemed an abominable sin, as also to kill infants newly born. And more powerful with them are good manners, than with other people are good laws.
It’s pretty clear that Tacitus was describing Germanic domestic relations to provide a contrast with Rome’s. One can deduce typical Roman behavior from what the Germans do not do. The Germans took marriage and chastity seriously, and didn’t fool around. They are not promiscuous, adultery is rare, they do not practice abortion, etc. The implication, of course, is that these are all common practices in Rome.