(CG 3/125). That Marriage ought not to take place between Kindred (Book 3. God, the End of Creatures) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

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THE members of the body being the instruments of the soul, the end of every member is the use of it, as in the case of any other instrument. But there are members of the body the use of which is for the intercourse of the sexes: that therefore is their end. But that which is the end of any natural thing cannot be in itself evil, because the things of nature are ordinances of divine providence. Therefore the intercourse of the sexes cannot be in itself evil.

3. Natural inclinations are put into things by God, who is the prime mover of all. Therefore it is impossible for the natural inclination of any species to be directed to an object in itself evil. But in all full-grown animals there is a natural inclination to sexual union, which union therefore cannot be in itself evil.

4. That without which something good and excellent cannot be, is not in itself evil. But the perpetuity of the species in animals is not preserved except through generation, which is of sexual intercourse.

Hence it is said: She sinneth not, if she marry (1 Cor. vii, 36).

Hereby is excluded the error of those who totally condemn marriage, which some do because they believe that temporal things proceed not from a good but from an evil principle.