Lactantius, answered from the texts and cited to the page.
I will take your three questions in the order you set them, for they are one argument seen from three sides. On the first: let each man set before himself that the union of the two sexes was given to living creatures for the sake of generation, and that the law laid down in these affections is that they may provide succession. As God gave us eyes not that we might gaze and take pleasure but that we might see for the needs of life, so the genital part, as its very name teaches, we have received for no other cause than the production of offspring.1
Yet I do not pretend that every man will be able to hold to this. Whoever cannot curb these affections, let him confine them within the prescribed limits of the lawful bed, so that he obtains what he eagerly desires and does not fall into sin; and if some necessity forbids even this, then the greatest virtue must be summoned, that continence may resist desire.2
The same purpose I read in the ordinance itself: for the preservation of each kind God contrived two sexes, joined by the allurements of pleasure, lest the condition of mortality should extinguish every kind of living creature.3 What I condemn is the profane man who turns that admirable work to base gain or obscene lust, and seeks from a most sacred thing nothing but an empty and barren pleasure.4
On the second, the husband owes exactly what the wife owes. The public law of Rome calls her alone an adulteress who has another man, and acquits the husband though he has several women. The divine law joins two in marriage into one body with equal right, and whoever has torn the union asunder and divided it among different persons is held an adulterer.5
It is unjust to demand what you cannot furnish yourself; this injustice has made adulteries, for women take it ill to keep faith with men who show them no mutual affection, and there is no adulteress so abandoned that she does not plead that by sinning she returns an injury rather than inflicts one.5 Quintilian said it excellently: the man who neither abstains from another's marriage nor guards his own cannot, being busy corrupting the wives of others, find leisure for domestic sanctity, and his wife, stirred by his very example, thinks she is imitating him or avenging herself.5
I add that even the thought is not exempt, for chastity is praised in man because it is voluntary and not natural.5 On the third, you ask of the womb. I did write that the vein in males which holds the seed is double, the right containing male seed and the left female, and that in the whole body the right part is masculine and the left feminine; and in the woman the womb divides into two parts, wound about like a ram's horns, the part turned to the right masculine, that turned to the left feminine.6
From this I drew that if feminine seed flows into the right part, a female is produced, but one with something of manliness beyond what her sex permits, whether strong limbs, immoderate height, a rough face, a robust voice, or a bold spirit; and if the masculine seed reaches the right and the feminine the left, both offspring come forth rightly.7
I took this from Varro and Aristotle, and I said of their account of conception that these things have perhaps been gathered from abortions.6 I offered it as the opinion of the physicians, hedged as such, and I say plainly that how the seed gathers and grows together the human mind cannot comprehend. What I held without hedging was the design: that more strength was given to males, that women might the more easily be brought to bear the marriage yoke.7
Better instruments than those of my age may weigh the embryology differently, and the providence I praised in the body would be no less wonderful for it. Continence stays the summit of the virtues, offered and not commanded. God knows what necessity He laid on these affections, and the man who can strive towards it He will acknowledge as a disciple, and he shall be like God.8
God joined the sexes for generation and set desire in them so that kinds might continue, and gave man alone the virtue to rule it. Whoever cannot be continent must keep desire within the lawful bed.9 Marriage makes two one body with equal right, so the husband who goes to another woman is as much an adulterer as the wife, whatever the public law says; the mind that covets is already adulterous.
The adversary invented the brothel and intercourse against nature, which profane the work God made. Continence is the summit of the virtues, offered and not commanded. In the womb the right side makes the male and the left the female, and the divine design is wonderful in both.10
Each one ought therefore to set before himself that the union of the two sexes has been given to living creatures for the sake of generation, and that the law laid down in these affections is that they may provide succession. But as God gave us eyes, not that we might gaze and take pleasure, but that we might see for the sake of those actions which pertain to the necessity of life, so the genital part of the body, as the name itself teaches, we have received for no other cause than the production of offspring.Passages left in Latin by ANF, Inst. VI.23 and Opif. XII-XIII
Whoever cannot curb those affections, let him confine them within the prescribed limits of the lawful bed, that he may both obtain that which he eagerly desires, and yet not fall into sin.Passages left in Latin by ANF, Inst. VI.23 and Opif. XII-XIII
But how wonderful is that very ordinance of God, that for the preservation of each kind He contrived two sexes, male and female, who, being joined together by the allurements of pleasure, might bring forth successive offspring, lest the condition of mortality should extinguish every kind of living creature.Passages left in Latin by ANF, Inst. VI.23 and Opif. XII-XIII
impious and profane men commit the utmost wickedness, who turn the divine and admirable work of God... either to the basest gains or to the shameful works of obscene lust, so that now they seek nothing else from a most sacred thing than an empty and barren pleasure.Passages left in Latin by ANF, Inst. VI.23 and Opif. XII-XIII
But the divine law so joins two in marriage, which is into one body, with equal right, that whoever has torn asunder the union of the body, and divided it among different persons, is held an adulterer.Passages left in Latin by ANF, Inst. VI.23 and Opif. XII-XIII
But the right vein contains the male seed, and the left the female; and in general, in the whole body the right part is masculine, and the left feminine.Passages left in Latin by ANF, Inst. VI.23 and Opif. XII-XIII
Likewise, if the seed of the feminine sex has flowed into the right part, a female is indeed produced; but since she was conceived in the masculine part, she has in herself something of manliness beyond what the nature of her sex permits.Passages left in Latin by ANF, Inst. VI.23 and Opif. XII-XIII
This kind of continence is as it were the summit and the consummation of all virtues.Passages left in Latin by ANF, Inst. VI.23 and Opif. XII-XIII
Since, therefore, lust produces these works, and these deeds are contrived, we must be armed against it with the greatest virtue. Whoever cannot curb those affections, let him confine them within the prescribed limits of the lawful bed, that he may both obtain that which he eagerly desires, and yet not fall into sin. For what do abandoned men want?Passages left in Latin by ANF, Inst. VI.23 and Opif. XII-XIII
Lastly, let us strive to undergo death itself, not unwillingly or timidly, but willingly and undauntedly, as those who know what glory we are about to have in the presence of God, having triumphed over the world and coming to the things promised us; with what good things and how great blessedness we shall be compensated for these brief evils of punishments, and the injuries of this life.The Epitome of the Divine Institutes
The pages his Victorian translator left in Latin, now in English; the single standard of fidelity is his most modern claim, the condemnations and the embryology his least.