Why is incest wrong?

[Jack3]

Title says all.
Of course I accept the Divine law, I'm only looking for reasons
I tried to search by myself, in the Summa.
https://www.newadvent.org/summa/3154.htm#article9

Now sexual intercourse with women related by consanguinity or affinity is unbecoming to venereal union on three counts.
First, because man naturally owes a certain respect to his parents and therefore to his other blood relations, who are descended in near degree from the same parents: so much so indeed that among the ancients, as Valerius Maximus relates [Dict. Fact. Memor. ii, 1, it was not deemed right for a son to bathe with his father, lest they should see one another naked. Now from what has been said (II-II:142:4; II-II:151:4), it is evident that in venereal acts there is a certain shamefulness inconsistent with respect, wherefore men are ashamed of them. Wherefore it is unseemly that such persons should be united in venereal intercourse. This reason seems to be indicated (Leviticus 18:7) where we read: "She is thy mother, thou shalt not uncover her nakedness," and the same is expressed further on with regard to others.

The second reason is because blood relations must needs live in close touch with one another. Wherefore if they were not debarred from venereal union, opportunities of venereal intercourse would be very frequent and thus men's minds would be enervated by lust. Hence in the Old Law [Leviticus 18] the prohibition was apparently directed specially to those persons who must needs live together.

The third reason is, because this would hinder a man from having many friends: since through a man taking a stranger to wife, all his wife's relations are united to him by a special kind of friendship, as though they were of the same blood as himself. Wherefore Augustine says (De Civ. Dei xv, 16): "The demands of charity are most perfectly satisfied by men uniting together in the bonds that the various ties of friendship require, so that they may live together in a useful and becoming amity; nor should one man have many relationships in one, but each should have one."

Aristotle adds another reason (2 Polit. ii): for since it is natural that a man should have a liking for a woman of his kindred, if to this be added the love that has its origin in venereal intercourse, his love would be too ardent and would become a very great incentive to lust: and this is contrary to chastity. Hence it is evident that incest is a determinate species of lust.

While this makes the case that incest is risky or that non-incestuous marriages are better, it does not establish them as absolute evil under the natural law.
I can conceive of a man who remains respectful, has many friends, wants to marry a relative who is not living nearby, and would not love each other too ardently - such that his reasons would not apply.

[Highlander]

I think that the evolutionary argument is convincing.

[peregrinator]

While this makes the case that incest is risky or that non-incestuous marriages are better, it does not establish them as absolute evil under the natural law.

Because they are not an absolute evil under the natural law. If they were then how did Cain, Seth, etc. have families? I think only unions that are consanguinous in the direct line are forbidden by the natural law.

https://www.newadvent.org/cathen/04264a.htm

[Jack3]

So, under the natural law, you cannot marry your ancestors but you can marry cousins,amirite? What about siblings then?

[peregrinator]

So, under the natural law, you cannot marry your ancestors but you can marry cousins,amirite? What about siblings then?

I think that's answered above. Siblings are not related in the direct line so unions between them are not forbidden by the natural law. If they were then the propagation of the human race involved practices contrary to the natural law by necessity, and I don't think God would have or even could have ordained that. But the Church would never grant a dispensation for this absent some really exceptional circumstance.

[peregrinator]

I think the article from the Catholic Encyclopedia is very helpful on this topic. One of the reasons the Church was so strict about this in the middle ages (far more than it is today) was to unite the peoples of Christendom under the banner of Christ and His Church.

[Highlander]

During the Middle Ages, as I recall, one motive of the Church temporal was to move resources from economically powerful extended families to the Church. Familial intermarriage kept resources within the family; thus, intermarriage was common. Reducing the scope and breadth of such marriages provided the Church temporal with the opportunity to obtain certain researches as family branches were extinguished by death.

[peregrinator]

Sounds cynical to me, like something a certain trad Anglican would say

[Obi-Wan Kenobi]

Certain resources?

[Highlander]

Property and money.

Cynical or not, intended or not, the medieval Church temporal had motives and practices a good bit different than today. The Church temporal, by expanding the definition of consanguinity, narrowed the pool of available spouses and diminished the power of large, extended, interrelated clans. One effect was to restrict inheritance and channel some of the transfer of wealth from the family to the Church.

[Obi-Wan Kenobi]

Your original post said "researches" instead of "resources".

[Highlander]

That is very me. OTOH, the Irish were keeping civilization alive and the precursors to the universities were forming ... so maybe it was researches.