Despite Ladislaus's quoting
Casti Connubii, which says "so long as they are subordinated to the primary end," it seems he's arguing for the "secondary means" without any consideration of the primary end (which he seems to think can cease to exist).
Being infertile, for example, does not annihilate the primary end of the act.
Actively contracepting, e.g., is to disregard the primary end.
It seems Ladislaus thinks the primary end ceases to exist, yet
it is always there,* even for those contracepting. It's one's disregard or regard of that end that can make the act sinful or not.
(*although some here may argue it's not there during pregnancy; that's the whole controversy of this thread)In "
Whether it is a mortal sin for a man to have knowledge of his wife, with the intention not of a marriage good but merely of pleasure?," St. Thomas doesn't flat-out say "yes" but makes distinctions:
Some say that whenever pleasure is the chief motive for the marriage act it is a mortal sin; that when it is an indirect motive it is a venial sin; and that when it spurns the pleasure altogether and is displeasing, it is wholly void of venial sin; so that it would be a mortal sin to seek pleasure in this act, a venial sin to take the pleasure when offered, but that perfection requires one to detest it.
This seems similar to Amakusa's view. St. Thomas condemns it:
But this is impossible, since according to the Philosopher (Ethic. x, 3,4) the same judgment applies to pleasure as to action, because pleasure in a good action is good, and in an evil action, evil; wherefore, as the marriage act is not evil in itself, neither will it be always a mortal sin to seek pleasure therein.
Then he shows, when "the goods of marriage" (e.g., the primary end) are disregarded, in what cases the act is a venial or mortal sin:
Consequently the right answer to this question is that if pleasure be sought in such a way as to exclude the honesty of marriage, so that, to wit, it is not as a wife but as a woman that a man treats his wife, and that he is ready to use her in the same way if she were not his wife, it is a mortal sin; wherefore such a man is said to be too ardent a lover of his wife, because his ardor carries him away from the goods of marriage. If, however, he seek pleasure within the bounds of marriage, so that it would not be sought in another than his wife, it is a venial sin.
St. Thomas does not contradict
Casti Connubii here because St. Thomas is discussing the case when a good of marriage (e.g., the primary end) is disregarded, but
Casti Connubii is only considering the case "so long as they are subordinated to the primary end" ("and so long as the intrinsic nature of the act is preserved"). Thus, St. Thomas implies it is sinful to disregard
any end of marriage, the primary or secondary; e.g., Henry VIII sinned by disregarding the secondary ends, and contraceptors sin by disregarding the primary end.
St. Thomas says if pleasure is sought in marriage so as to avoid adultery, it is a venial sin. Amakusa argues it is always at least a venial sin, which is false.
St. Thomas's teaching is beautiful; it is very balanced and far from having any tint of puritanism or dualism. His distinction between a man treating his wife as a wife vs. as simply a woman is also excellent.
His answer also seems to answer the question of this thread: It would "exclude the honesty of marriage" to ask a pregnant wife, especially one advanced in pregnancy, to pay the debt. Hence, a husband would sin in doing so, esp. if there is risk of abortion.