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WARNING!! This question and answer is of a mature nature. Parental discretion is advised.

Dear Jared:

There is nothing contradictory about this when you understand the theology of sexual expression.

Pope John Paul II, in his Theology of the Body does not specifically mention oral sex but he does make it clear that that when spouses act to merely satisfy the sexual needs of the other there is damage to the communion that is suppose to be affirm in the marital embrace.

It can happen that one of the two persons exists only as the subject of the satisfaction of the sexual need, and the other becomes exclusively the object of this satisfaction.  This does not correspond to the union or personal communion to which man and woman were mutually called from the beginning.  On the contrary, it conflicts with it.  Moreover, the case in which both the man and woman exist reciprocally as the object of satisfaction of the sexual need, and each on his or her part is only the subject of that satisfaction, does not correspond to this unity of communion.  On the contrary, it clashes with it.  This reduction of such a rich content of the reciprocal and perennial attraction of human persons in their masculinity or femininity does not at all correspond to the ‘nature’ of the attraction in question.  This reduction extinguishes the personal meaning of communion, characteristic of man and woman.

—John Paul II, The Theology of the Body: Human Love in the Divine Plan (Boston, MA: Pauline Books & Media, 1997), 152

Pope Pius XII wrote:

Marriage is a mutual commitment in which each side ceases to be autonomous, in various ways and also sexually: the sexual liberty in agreement together is great; here, so long as they are not immoderate so as to become slaves of sensuality, nothing is shameful, if the complete acts - the ones involving ejaculation of the man's seed - that they engage in are true and real marriage acts."

—Pope Pius XII, "Address to the Second World Congress on Fertility and Sterility, " May 19, 1956 (AAS, 48.473).

Oral sex outside of foreplay or afterplay in which the man ejaculates outside of his wife's vagina is not morally permissible.

The marital embrace is meant to be a mutual self-giving. That goal fails if the couple uses each other as mere sexual objects to receive sexual pleasure. Oral sex for its own purpose is not a mutual self-giving, it is "giving" only. In effect, it is a form of masturbation, that is, the spouse masturbating the other. Mutual self-giving is not achieved in this way.

"Now, Lord, You know that I take this wife of mine not because of lust, but for a noble purpose." —Tobit 8:7

The reason this is tied to the man is that a man's climax is inherently directed to the possibility of new life. The man is the progenitor of new life. Therefore, it is never permissible for him to "spill his seed" outside the vagina of his wife. The wife's orgasm is not so linked to life.

This is the case even if pregnancy is not possible. There are two purposes of sex, procreation and the bonding of the spouses in a mutual self-giving. The mutual self-giving and bond that is one of the purposes of marital sex is not accomplished if the man ejaculates outside of his wife's vagina. The nature of mutual self-giving and the bond created by the marital embrace remains intact even when the purpose of procreation is not possible.

Even within marriage, not all sex acts are permitted; there is a kind of chastity in marriage. Any sex act tht degrades the human person is not permitted, such as sado-masochism, bondage, domination, rape fantasies, and the like.

The husband demanding sex when the wife does not wish it and the wife using sex as a weapon to withhold to manipulate her husband also degrades the human person and violates the mutual self-giving that the marital embrace is meant to be.

Marital sex is a holy act when does with marital chastity. It is a sublime image of the relationship between Christ and His Church. This inherent purpose of sex that God created sex to be is part of why certain sects, like Satanist, use sex as a form of worship. This is perverted but it does relate to the sublime reality of the sexual faculity that God meant only for marriage.

In today's world, most of the world worships sex. Sex is more important that anything else. That is why couples will marry outside of the Church if the Church says they are not eligible for marriage. These couples apparently are willing to risk their souls to hell rather than do the right thing. Some do not marry at all and live in concubinage and risk their souls to hell in that way.

None of this is what sex is to be. Sex in marriage is to be an embrace that is mutually self-giving, not mutually stimulating.

To behave morally within marriage should not be a problem. There is no reason why a man cannot always ejaculate in his wife's vagina. If there is some medical reason why that is not possible, then abstinance must be practice. If one cannot abstain, that is a sure sign that the person has a perverted view of sex, seeking sexual pleasure for its own sake, and has a deep moral problem of lacking all four cardinal virtues of prudence, justice, temperance (restraint), and fortitude (courage). 

St. Paul instructs us:

(1 Cor 7:3-5)  The husband should give to his wife her conjugal rights, and likewise the wife to her husband. For the wife does not rule over her own body, but the husband does; likewise the husband does not rule over his own body, but the wife does. Do not refuse one another except perhaps by agreement for a season, that you may devote yourselves to prayer; but then come together again, lest Satan tempt you through lack of self-control. 

Do not allow Satan to tempt you through lack of self-control, to violate the virtues of prudence, justice, temperance, and fortitude. Satan would like nothing better than to get us to lose virtue.

The marital embrace is to be virtuous.

God Bless,
Bro. Ignatius Mary