Now, everybody hold on, and read my whole response before you object! St. Alphonsus and all the traditional moral theologians of the Church do not allow what is called “oral sex” as part of foreplay for natural sexual intercourse. This is for two reasons—one practical, the other moral.
The first and practical reason, admitted by all the traditional authors, is that there is a concrete danger of completing the act before natural intercourse can begin. The similarity of the stimulation caused by oral relations to natural intercourse makes it likely that from time to time, at least, if a couple uses this means of stimulation, the man will ejaculate before natural intercourse. This would make the relations non-procreative, and thus wrong. Perhaps there are couples for whom this would never be the case, but the second reason is the more profound from the human male-and-female point of view.
The second and moral reason is that, unfortunately, a couple may develop a concrete preference for oral stimulation over natural sexual intercourse—what St. Alphonsus calls an affectus sodomiticus, an attraction to sodomitical sex. So they get into the situation emotionally of preferring an unnatural act and having the natural one only because they are morally bound to. This is all the more common in our time where many young people engage in oral sex exclusively so as to avoid conception, and once they are married, they end up preferring it.
People sometimes prefer oral stimulation because it gives them more pleasure, but the pleasure is not the measure of what is to be preferred in sexual relations but rather the standard is what makes the couple relate to each other in the integrity of the marital act. This means that they should train their hearts to prefer face to face (not face to groin!) communication in the expression of their love and should prefer that use of their genitals that is directly related to human procreation, not some kind of elaborate acrobatics that ludicrously overemphasizes the pleasure of one or the other.
This being said, even the relatively (but not completely) austere St. Alphonsus allows oral contact with the spouse’s genitals obiter, that is, “in passing,” as a brief expression of reverence or affection without oral penetration. That ought to be romantic enough for anyone and would avoid developing an affection for an unnatural act.
Whether this prohibition is gravely binding or not is another question. If the first reason is the issue, then the practice of oral stimulation is gravely wrong. Period. If the second reason is the issue, the matter may be venial, but this does not change the answer regarding what is permitted, since we are not permitted to deliberately commit a venial sin.
Of course, the man may help his spouse manually to reach completion, as long as he does before or after her and in the natural manner.
There are, unfortunately, some authors today, with the reputation for orthodoxy, who teach that this practice is licit. They are wrong. The whole tradition of moral theology and care of souls is against them. They can establish elaborate rationales based on an abusive interpretation of St. John Paul’s theology of the body, but in the end, the question is: what kind of things should Christians desire to do with their bodies? What do my outward gestures symbolize? What expresses the dignity of the marital relationship most clearly?
In any case, there is an extrinsic argument in the fact the Magisterium has taught in the past that the concrete conclusions of St. Alphonsus are always reliable and may be followed, even if one does not agree with his rationale. This is a safer route, and a safer route is the better by far in a matter so important and delicate about which we may be inclined to deceive ourselves.