The Julian marriage laws (nos. 120-123, etc.)
In 18 B.C., the Emperor Augustus turned his attention to social problems at Rome. Extravagance and adultery were widespread. Among the upper classes, marriage was increasingly infrequent and, many couples who did marry failed to produce offspring. Augustus, who hoped thereby to elevate both the morals and the numbers of the upper classes in Rome, and to increase the population of native Italians in Italy, enacted laws to encourage marriage and having children (lex Julia de maritandis ordinibus), including provisions establishing adultery as a crime.
The law against adultery made the offence a crime punishable by exile and confiscation of property. Fathers were permitted to kill daughters and their partners in adultery. Husbands could kill the partners under certain circumstances and were required to divorce adulterous wives. Augustus himself was obliged to invoke the law against his own daughter, Julia, and relegated her to the island of Pandateria. 
The Augustan social laws were badly received and were modified in A.D. 9 by the lex Papia Poppaea, named for the two bachelor consuls of that year. The earlier and later laws are often referred to in juristic sources as the lex Julia et Papia.
In part as a result of Christian opposition to such policies, the laws were eventually nearly all repealed or fell into disuse under Constantine and later emperors, including the emperor Justinian. Only the prohibitions against intermarriage, as that between senators and actresses, remained.
The first three of the texts that follow do not come from the Roman jurists but give background for the passing of the laws. The remaining texts in this section are from legal works interpreting the provisions of this legislation by a number of jurists. The juristic sources are also our best source for the actual provisions of the laws.
Speech of the censor Quintus Caecilius Metellus Macedonicus  about the law requiring men to marry in order to produce children. According to Livy (Per. 59), in 17 B.C. Augustus read out this speech, which seemed "written for the hour", in the Senate in support of his own legislation encouraging marriage and childbearing (see no. 121).
If we could survive without a wife, citizens of Rome, all of us would do without that nuisance; but since nature has so decreed that we cannot manage comfortably with them, nor live in any way without them,  we must plan for our lasting preservation rather than for our temporary pleasure.
121. Prizes for marriage and having children. Rome, 1st cent.
A.D. (Dio Cassius, History of Rome 54.16.1-1. Early 3rd
cent. A.D. G)
[Augustus] assessed heavier taxes on unmarried men and women without husbands, and by contrast offered awards for marriage and childbearing. And since there were more males than females among the nobility, he permitted anyone who wished (except for senators) to marry freedwomen, and decreed that children of such marriages be legitimate.
122. Augustus' law. Rome, 18 B.C. (Suetonius, Life of
Augustus 34. L)
He reformed the laws and completely overhauled some of them, such as the sumptuary law, that on adultery and chastity, that on bribery, and marriage of the various classes.
Having shown greater severity in the emendation of this last than the others, as a result of the agitation of its opponents he was unable to get it approved except by abolishing or mitigating part of the penalty, conceding a three-year grace-period (before remarriage) and increasing the rewards (for having children).
Nevertheless, when, during a public show the order of knights asked him with insistence to revoke it, he summoned the children of Germanicus,  holding some of them near him and setting others on their father's knee; and in so doing he gave the demonstrators to understand through his affectionate gestures and expressions that they should not object to imitating that young man's example.
Moreover, when he found out that the law was being sidestepped through engagements to young girls  and frequent divorces, he put a time limit on engagement and clamped down on divorce.
123. The consequences of adultery (Paul, Opinions 2.26.1-8,
10-12, 14-17. L)
2.26 (1) In the second chapter of the lex Julia concerning adultery, either an adoptive or a natural father is permitted to kill with his own hands an adulterer caught in the act with his daughter in his own house or in that of his son-in-law, no matter what his rank may be.
(2) If a son under paternal power, who is the father, should surprise his daughter in the act of adultery, while it is inferred from the words of the law that he cannot kill her, still, he ought to be permitted to do so.
(3) Again, it is provided in the fifth chapter of the lex Julia that it is permitted to detain an adulterer who has been caught in the act for twenty hours, calling neighbours to witness.
(4) A husband cannot kill anyone taken in adultery except persons who are infamous, and those who sell their bodies for gain, as well as slaves. His wife, however, is excepted, and he is forbidden to kill her.
(5) It has been decided that a husband who kills his wife when caught with an adulterer should be punished more leniently, for the reason that he committed the act through impatience caused by just suffering.
(6) After having killed the adulterer, the husband should at once dismiss his wife, and publicly declare within the next three days with what adulterer, and in what place he found his wife.
(7) A husband who surprises his wife in adultery can only kill the adulterer when he catches him in his own house.
(8) It has been decided that a husband who does not at once dismiss his wife whom he has taken in adultery can be prosecuted as a pimp.
(10) It should be noted that two adulterers can be accused at the same time with the wife, but more than that number cannot be.
(11) It has been decided that adultery cannot be committed with women who have charge of any business or shop. 
(12) Anyone who has sexual relations with a free male without his consent shall be punished with death.
(14) It has been held that women convicted of adultery shall be punished with the loss of half of their dowry and the third of their goods, and by relegation to an island. The adulterer, however, shall be deprived of half his property, and shall also be punished by relegation to an island; provided the parties are exiled to different islands.
(15) It has been decided that the penalty for incest, which in case of a man is deportation to an island, shall not be inflicted upon the woman; that is to say when she has not been convicted under the lex Julia concerning adultery.
(16) Sexual intercourse with female slaves, unless they are deteriorated in value or an attempt is made against their mistress through them, is not considered an injury.
(17) In a case of adultery a postponement cannot be granted.
15. For an interesting fictional account of Julia's exile and the events which led to it, see [Williams, 1972 #137].
16. This portion of the speech is preserved by Aulus Gellius (1.6.2), who mistakes this Metellus (Macedonicus, cos. 143 B.C.) for Metellus Numidicus (cos. 109 B.C.). Cf. [Holford-Strevens, 1988 #116], 65 n.20, 228.
17. Cf. Aristophanes, Lysistrata 1038-39: 'A true saying and well-said: you can't live with the cursed creatures or without them'.
18. I.e., his great-grandchildren, the children of Germanicus and his granddaughter Agrippina.
19. To postpone marriage and the birth of children.
20. Not because is is prohibited but in the sense that sexual relations with such women are not considered adultery.