St Alphonsus Ligouri on relations during menstruation

 Here is what I find in Liguori, VI, 925:

The moralists cited by St. Alphonsus distinguish between natural, ordinary menstrual flux (i.e., that which takes 2-3 days) and "extraordinary flux" that can last up to twelve days or more. Conjugal relations during extraordinary flux is always licit.

In fluxumenstruoruminnaturali non estprohibitumadmenstruatamaccedere in lege nova, tum propter infirmitatem, quiamulie in talistatuconcipere non potest; tum quiatalisfluxusestperpetuus et diuturnus, undeoportet, quod virperpetuoabstineret.

The reasons given for liceity is that a woman cannot conceive in such a state (unlike natural menstruation, when it was believed she can), and also because it can be so prolonged that it would require constant abstinence by the man. The Old Law's rigid prescription against relations during any flux of blood is not applicable to Christians.

As for natural, ordinary menstrual flux, there are three opinions:

First, some, including Scotus, hold that it is a mortal sin. Reasons: (1) Leviticus 20:18. Others object that the above prescription is a ceremonial law, not applicable to Christians. (2) Canon of Pope Gregory prohibiting this. (3) This increases the risk of the child being born leprous or monstrous, and there is a danger of pouring seed in vain, since it is rarely or never useful for conception at that time.

Second, some say there is no guilt whatsoever in this. Reasons: There is no law currently in force that forbids this. Divine law: St. Jerome in his commentary on Ezekiel says the prohibition in Leviticus was because at that time it could cause leprous infection to the fetus. Today it is certain that women cannot conceive during menstruation.  Church law: A gloss on Pope Gregory's term "prohibeantur" says "it is discouraged under the form of a prohibition." It is not against natural law, even though there is danger of frustrating the seed. It is not necessary for morally upright conjugal relations (coitum cohonestandum)  that from it follows generation (ut ex eosequaturgeneratio), but it suffices that that the act is, of itself, fit for generation (ille per se sit aptusgeneratione),  even if it is by accident (per accidens) scattered, as in the case of a pregnant or sterile woman.

The third and most common opinion, which Liguori considers more probable, is that coition during menstruation is a venial sin. The reasons given in the second opinion suffice to show that it is not a mortal sin. Nonetheless, it is a venial sin because such partners are unwilling to wait for a more suitable time that is coming soon; the woman currently being unfit for the retention of seed and less suitable to the purpose of generation.

Note that, in this majority opinion, such practice is not considered contraceptive (for then it would be a mortal sin), even though the chance of procreation is known to be negligible. What makes it a venial sin is that the partners have so little control of their lust that they cannot even wait briefly for a more apt time coming soon (et jam breviadventurum). This finding of venial sin would not apply to cases of prolonged accidental infertility.