St. Augustine condemns NFP.

[Geremia]

Couples sin who mutually practice NFP to avoid conception.

It can be that one spouse must pay the marital debt in an infertile time to prevent the other spouse from sinning, but the one paying the debt doesn't sin in that case.

[andy]

Couples sin who mutually practice NFP to avoid conception.

It can be that one spouse must pay the marital debt in an infertile time to prevent the other spouse from sinning, but the one paying the debt doesn't sin in that case.

You are still an accessory spouse's sin though, which is a sin.

[Geremia]

You are still an accessory spouse's sin though, which is a sin.

Materially, yes; not necessarily formally
The same reasoning goes for when one spouse commits onanism against the will of the other.

St. Thomas, suppl. q. 49 a. 5 ad 2:

If a man intends by the marriage act to prevent fornication [or any other sin] in his wife, it is no sin, because this is a kind of payment of the debt that comes under the [marriage] good of "faith [fidelity]."

[AlNg]

St. Augustine is also making the point that marriage with no intention to have children is not true marriage. 

Suppose an elderly widow and an elderly widower get married in the Church. Is their marriage invalid if they have no intention of having children because the wife is beyond the child bearing age? If so, are they then living in a state of adultery even though they were married in the Church? 

[Michael Wilson]

The elderly couple do not plan to have any children, but neither do they deliberately exclude the having of children in the marital act; as opposed to those of child-bearing age who would enter into marriage with this intention.
A marriage where the couple would also at the same time make a vow of chastity, would also be a valid marriage, because the couple are not impeding the marital act.

[andy]

You are still an accessory spouse's sin though, which is a sin.

Materially, yes; not necessarily formally
The same reasoning goes for when one spouse commits onanism against the will of the other.

St. Thomas, suppl. q. 49 a. 5 ad 2:

If a man intends by the marriage act to prevent fornication [or any other sin] in his wife, it is no sin, because this is a kind of payment of the debt that comes under the [marriage] good of "faith [fidelity]."

So if a wife asks his husband to fulfill the marital debt and he puts a condom on, she is free of sin?

[Michael Wilson]

Here "Moral Theology V-II McHugh and Callan: (translated from the Latin text in the book)

Cooperation of the Wife in Onanism or Contraception.
a). Formal cooperation is gravely unlawful, since it includes the approval of my own sin. Hence the wife commits a serious sin who, with her questions about the troubles of pregnancy, forces her husband to have an onanistic encounter, or who does not sometimes try to dissuade him from the wrongful plan of having an onanistic encounter, or who actively aids the breaking of the copulation, or who inwardly rejoices in his own sin
b) Material cooperation for onanism for a serious reason (e.g., based on fear of a quarrel, abusive cohabitation, adultery of a husband) is allowed; for the action of a woman, viz. to have a natural copulation is honorable, and there is a sufficient reason to permit the abuse committed by a partner (1515 sqq.) Yes, a wife can claim debt from her onanistic husband, if otherwise she would be forced to abstain for a long time from all the use of marriage with the risk of incontinence, because love for the husband obliges one to abstinence with so much inconvenience.
c). Merely material cooperation for contraception does not seem possible; for the contraceptive couple is intrinsically and from the beginning evil (1517, 1527). Wherefore the wife is not allowed to claim the debt, nor to have it passively. But at every turn they are forced to resist positively for their strength. If, however, there is a very serious reason for permitting the act, as, for example, the danger of death, it must be opposed only by the resistance to which the oppressed virgin is bound (2497), with consent evidently denied (See Irish Ecclesiastical Record, June, 1940, pp. 634 ff., and March, 1948, pp. 244 ff.)

[LausTibiChriste]

Women got it easy

[Geremia]

So if a wife asks his [sic!] husband to fulfill the marital debt and he puts a condom on, [then they are not engaging in a marriage act.]

Sodomy is not the marriage act.
Onanism is coitus interruptus.

[andy]

So if a wife asks her husband to fulfill the marital debt and he puts a condom on, [then they are not engaging in a marriage act.]

Sodomy is not the marriage act.
Onanism is coitus interruptus.

Sorry for the mistake - fixed and the graphic text. I make silly clerical errors like this in all languages I use. A brain defect I guess.

Yes I totally agree with then they are not engaging in a marriage act in this case.