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Contraception is not OK according to the Catholic Church, with the exception of fertility awareness methods (FAMs), which involve abstinence when a woman is in the part of her cycle where she is fertile.

However, there are ways to decrease male fertility, such as getting 'snipped' (a vasectomy), which the Catholic Church says constitutes a mortal sin. A Catholic.com article states

"A vasectomy constitutes grave matter. Together with full knowledge of the gravity of the action and full and free consent to the action, a mortal sin is committed."

What about simply eating in a way that is intended to decrease male fertility, such as going vegan and eating lots of soy, say - according to the Catholic Church, would that also be sinful, and if so, would it constitute a mortal sin or would it be a venial sin?

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St. Alphonsus of Liguori, Theologia Moralis lib. 6, 942, p. 690:

A husband or wife rendering himself or herself impotent, even by licit means (e.g., fasts, etc.), sins gravely. If one cannot otherwise perform the [marriage] debt, it is licit to not observe the fasts of the Church.
Peccat graviter vir vel mulier, si se impotentem reddat, etiam mediis alias licitis, v.gr. ieiuniis etc. invita comparte: imo si aliter non posssit reddere debitum, licite non observat ieiunia ecclesiæ.

cf. ibid. lib. 3, 1034 §4, p. 768:

But if the man observing the fasts of the Church, and other moderate [fasts] from devotion, is rendered less potent to render the [marriage] debt, he is not bound to refrain from them; because he is not bound to make himself more potent by lacking spiritual [goods].
Si autem vir servans ecclesiæ ieiunia, et alia moderata ex devotione, redderetur minus potens ad debitum reddendum, non teneretur ab eis abstinere; quia non tenetur cum spirituali iactura illa omittere ut potentiorem se reddat

Impotency ≠ infertility, but they are related. An impotent husband is infertile, but an infertile husband isn't necessarily impotent.

  • Impotence (impotentia coeundi) means the inability to perform the marital act.
    A husband or wife invalidly marry if they know they cannot perform the marital act; impotency is diriment impediment to marriage.

  • Infertility or sterility (impotentia generandi) means a child does not result from a marital act.

Now, if one intentionally eats a diet to render himself impotent to the extent he becomes infertile, but is still able to perform the marital act, that is no different than artificial contraception, condemned in Casti Connubii §56:

any use whatsoever of matrimony exercised in such a way that the act is deliberately frustrated in its natural power to generate life is an offense against the law of God and of nature, and those who indulge in such are branded with the guilt of a grave sin.
quemlibet matrimonii usum, in quo exercendo, actus, de industria hominum, naturali sua vitæ procreandæ vi destituatur, Dei et naturæ legem infringere, et eos qui tale quid commiserint gravis noxæ labe commaculari.

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    Engaging in the act only at the time when pregnancy is unlikely is deliberately frustrating the natural power to generate life is it it?
    – Kris
    Aug 25 at 21:06
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    Only engaging in intercourse when it's least likely to result in procreation absolutely is a "deliberately frustrated" act. I would further argue that, by this logic, anyone known to have significantly reduced fertility can't get married at all. (Well, if "marriage debt" is a thing. At minimum, it would be sinful for such a person to engage in any sexual act.)
    – Matthew
    Aug 25 at 21:07
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    @Kris I think the point is that Casti Connubii forbids so-called 'natural' 'family planning' as promoted today, by forbidding anything which "deliberately" frustrates the ability to concieve — i.e. all contraception, whether it be 'artificial' or 'natural' (what's the difference if the effect is the same), including only having intercourse when you believe you are less likely to concieve. Aug 25 at 21:08
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    @SolaGratia Isn't the response here that there isn't an act which is deliberately frustrated? I.e., there is a lack of an act. Aug 25 at 22:22
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    @SolaGratia For more on "just reasons" I found a good EWTN article by a seminary professor of moral theology: Just Cause and Natural Family Planning Aug 26 at 2:36
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See Mark 7

14 And when he had called all the people unto him, he said unto them, Hearken unto me every one of you, and understand: 15 There is nothing from without a man, that entering into him can defile him: but the things which come out of him, those are they that defile the man. 16 If any man have ears to hear, let him hear. 17 And when he was entered into the house from the people, his disciples asked him concerning the parable. 18 And he saith unto them, Are ye so without understanding also? Do ye not perceive, that whatsoever thing from without entereth into the man, it cannot defile him; 19 Because it entereth not into his heart, but into the belly, and goeth out into the draught, purging all meats? 20 And he said, That which cometh out of the man, that defileth the man.

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    Thanks for this answer, it's an interesting and relevant verse. But presumably the sin would involve what's occurring in the husband's heart, here, no? It is not the technical act of eating something, but the intention, which flows from the heart. Aug 29 at 4:51

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