What is the Catholic teaching on oral sex and impotence?

Prior to the papacy of John Paul II, the Catholic Church taught that oral sex to the point of o*g**m is not permitted under any circumstances. It was considered immoral for the husband or the wife to perform any sexual act other than normal intercourse to climax.

The human person does not possess the right and power to the full exercise of the sexual faculty, directly intended, except when he performs the conjugal act according to the norms defined and imposed by nature itself. Outside of this natural act, it is not even given within the matrimonial right itself to enjoy this sexual faculty fully... What has been said up to this point concerning the intrinsic evil of any full use of the generative power outside the natural conjugal act applies in the same way when the acts are of married persons or of unmarried persons, whether the full exercise of the genital organs is done by the man or the woman, or by both parties acting together." Pope Pius XII, Address to the Second World Congress on Fertility and Sterility, 19 May 1956

John Paul's papacy updated this teaching and opened the door to considering sexual pleasure beyond normal intercourse, including female o*g**m, permissible -- except that the male must not intentionally ej*c*late outside of the woman's v***na. According to Christopher West, John Paul taught that:

If a man was able to o*g**m during sexual intercourse but his wife did not, he may bring his wife to o*g**m after intercourse in whatever way he chooses (manual or oral stimulation). The reverse, however, is prohibited. A man’s o*g**m is always tied to his fertility, so, therefore, the Church states that oral sex that would end with a male o*g**m outside of sexual intercourse is not permissible.

Not every single sexual act, per se, need be procreative, but during the sexual act, there needs to be openness to procreative activity.

Additional information and references are available here.

Canon Law and the Catechism both have something to say on impotence.

• If the impotence developed prior to the marriage, the marriage is invalid (Canon 1084 §1).

• If the impotence developed after the marriage was consummated, the marriage cannot be dissolved because of it (Canon 1061 §§1–2, Canon 1141).

• The Catechism insists that marriage is open to fertility.

  1652. By its very nature the institution of marriage and married love is ordered to the procreation and education of the offspring and it is in them that it finds its crowning glory."¹⁶⁰

        Children are the supreme gift of marriage and contribute greatly to the good of the parents themselves. God himself said: "It is not good that man should be alone," and "from the beginning (he) made them male and female"; wishing to associate them in a special way in his own creative work, God blessed man and woman with the words: "Be fruitful and multiply." Hence, true married love and the whole structure of family life which results from it, without diminishment of the other ends of marriage, are directed to disposing the spouses to cooperate valiantly with the love of the Creator and Savior, who through them will increase and enrich his family from day to day.¹⁶¹

  1653. The fruitfulness of conjugal love extends to the fruits of the moral, spiritual, and supernatural life that parents hand on to their children by education. Parents are the principal and first educators of their children.¹⁶² In this sense the fundamental task of marriage and family is to be at the service of life.¹⁶³

  1654. Spouses to whom God has not granted children can nevertheless have a conjugal life full of meaning, in both human and Christian terms. Their marriage can radiate a fruitfulness of charity, of hospitality, and of sacrifice.

All of this points to sex being used for the procreation of children. The Catechism is also a little more explicit on sex for its own sake. I'm not entirely sure that the concept of oral sex had reached the hallowed halls of the Vatican when Pope S John Paul II promulgated the Catechism in 1990, but since it's effectively a form of masturbation, particularly in the wide definition the Catechism uses, this appears to be the nearest text:

2352. By masturbation is to be understood the deliberate stimulation of the genital organs in order to derive sexual pleasure. "Both the Magisterium of the Church, in the course of a constant tradition, and the moral sense of the faithful have been in no doubt and have firmly maintained that masturbation is an intrinsically and gravely disordered action."¹³⁷ ... Here sexual pleasure is sought outside of "the sexual relationship which is demanded by the moral order and in which the total meaning of mutual self-giving and human procreation in the context of true love is achieved."¹³⁸

      To form an equitable judgment about the subjects' moral responsibility and to guide pastoral action, one must take into account the affective immaturity, force of acquired habit, conditions of anxiety, or other psychological or social factors that lessen or even extenuate moral culpability.

Consequently the answer is No: they are not permitted to engage in oral sex. There may be circumstances where culpability might be lessened, but that does not mean the practice is allowed.

The Church recognises through the Catechism that childlessness is a great burden.

2374. Couples who discover that they are sterile suffer greatly. "What will you give me," asks Abraham of God, "for I continue childless?"^163a and Rachel cries to her husband Jacob, "Give me children, or I shall die!"¹⁶⁴

2375. Research aimed at reducing human sterility is to be encouraged, on condition that it is placed "at the service of the human person, of his inalienable rights, and his true and integral good according to the design and will of God."¹⁶⁵

The Catechism goes on to consider IVF, which has been the subject of other questions here. However, if impotence is merely a matter of erectile dysfunction (as this question implies) then medical interventions to counter that and allow normal sex are not forbidden.

137 Persona humana 9 (Congregation for the Doctrine of the Faith)
138 Persona humana 9 (CDF)
160 Gaudium et Spes 48 §1; 50
161 GS 50 §1; cf Gen 2:18, Mt 19:4, Gen 1:28
162 Cf Gravissimum educationis 3
163 Cf Familiaris consortio 28
163a Gen 15:2
164 Gen 30:1
165 Donum vitae intro 2 (CDF)