Objection 1. It would seem that the sin is not aggravated by the fact that the aforesaid injuries are perpetrated on those who are connected with others. Such like injuries take their sinful character from inflicting an injury on another against his will. Now the evil inflicted on a man's own person is more against his will than that which is inflicted on a person connected with him. Therefore an injury inflicted on a person connected with another is less grievous.
Objection 2. Further, Holy Writ reproves those especially who do injuries to orphans and widows: hence it is written (Sirach 35:17): "He will not despise the prayers of the fatherless, nor the widow when she poureth out her complaint." Now the widow and the orphan are not connected with other persons. Therefore the sin is not aggravated through an injury being inflicted on one who is connected with others.
Objection 3. Further, the person who is connected has a will of his own just as the principal person has, so that something may be voluntary for him and yet against the will of the principal person, as in the case of adultery which pleases the woman but not the husband. Now these injuries are sinful in so far as they consist in an involuntary commutation. Therefore such like injuries are of a less sinful nature.
On the contrary, It is written (Deuteronomy 28:32) as though indicating an aggravating circumstance: "Thy sons and thy daughters shall be given to another people, thy eyes looking on [Vulgate: 'May thy sons and thy daughters be given,' etc.]."
I answer that, Other things being equal, an injury is a more grievous sin according as it affects more persons; and hence it is that it is a more grievous sin to strike or injure a person in authority than a private individual, because it conduces to the injury of the whole community, as stated above (I-II, 73, 09). Now when an injury is inflicted on one who is connected in any way with another, that injury affects two persons, so that, other things being equal, the sin is aggravated by this very fact. It may happen, however, that in view of certain circumstances, a sin committed against one who is not connected with any other person, is more grievous, on account of either the dignity of the person, or the greatness of the injury.
Reply to Objection 1. An injury inflicted on a person connected with others is less harmful to the persons with whom he is connected, than if it were perpetrated immediately on them, and from this point of view it is a less grievous sin. But all that belongs to the injury of the person with whom he is connected, is added to the sin of which a man is guilty through injuring the other one in himself.
Reply to Objection 2. Injuries done to widows and orphans are more insisted upon both through being more opposed to mercy, and because the same injury done to such persons is more grievous to them since they have no one to turn to for relief.
Reply to Objection 3. The fact that the wife voluntarily consents to the adultery, lessens the sin and injury, so far as the woman is concerned, for it would be more grievous, if the adulterer oppressed her by violence. But this does not remove the injury as affecting her husband, since "the wife hath not power of her own body; but the husband" (1 Corinthians 7:4). The same applies to similar cases. of adultery, however, as it is opposed not only to justice but also to chastity, we shall speak in the treatise on Temperance (154, 8).
Source: Summa Theologica (New Advent)