The late Father Al Lauer strongly counsels against oral sex and oral-genital stimulation (non-consummated oral sex) in this brief publication at Presentation Ministries.
First, Fr. Lauer says: "One practice that contributes to the weakening of marriages is oral sex as part of foreplay in sexual relations. By oral sex, I am referring to oral-genital contact between spouses."  Notice that he calls oral sex as foreplay, even without climax, a type of "oral sex". Any oral-genital contact between spouses, regardless of the purpose, is the same type of act: oral sex.
Then Fr. Lauer states the following:
"First, oral sex is not natural. It is contrary to natural law. If oral sex is OK, then are anal sex or nasal sex also OK? There must be some natural parameters to sexual expression. Otherwise, we have little reason not to permit homosexual activity or even sex between humans and animals. Something must be natural, and therefore other things unnatural." 
He says that oral sex, even without climax, is not nature -- i.e. it is an unnatural type of sex. For it is always "contrary to natural law". Now every act that is contrary to natural law is intrinsically evil. Some acts are immoral due to a bad intetion. But if the intention changes, then the act would be permissible. Some acts are immoral due to circumstances. But if the circumstances change, then the act becomes moral. But intrinsically evil acts are always wrong to knowingly choose, as they are immoral by the very nature of the act. All this is explained in the Catechism of the Catholic Church, in the section on morality. But the point is that all acts contrary to natural law, not only sexual acts, are intrinsically evil and always immoral.
Fr. Lauer is right to compare oral sex to anal sex, for both types of sexual acts are inherently non-procreative. By definition, an act that is not open to life, by the very nature of the act, is intrinsically evil.
Fr. John Hardon, S.J. writes that conjugal chastity includes "mutual respect of each otherís dignity, which forbids any unnatural sexual activity, or sodomy."  And he defines "Unnatural sins" as "those sexual acts from which conception is impossible."  Every inherently non-procreative sexual act is by definition unnatural and a type of sodomy.
Fr. Lauer goes on to say that we would have little reason to forbid homosexual activity or worse sexual sins, if we did not have some distinction between what is natural and moral versus what is unnatural and immoral. The distinction is based largely on whether the sexual act is inherently procreative. For God has ordained that marriage and sex be ordered primarily toward procreation. Pope Pius XII: "the primary end of marriage is the procreation and education of offspring." 
Finally, Fr. Lauer adds a practical note. He has been asked "to counsel over a hundred married couples about oral sex as foreplay. After some time of discernment, not one has maintained that oral sex was a true expression of love. The motivation behind oral sex is often lust." 
The sin of lust can be entirely interior, as when Jesus speaks of lust as a type of adultery of the heart (Mt 5:28). Then, too, unnatural sexual acts are inherently of lust, regardless of the motivation. For inherently disordered sexual acts are not fit for the expression of true love, but they are in conformity with the interior state that often accompanies unnatural sex: lust. A true loving marriage, especially the Catholic Christian Sacrament of holy Matrimony, should not make use of any unnatural sexual acts in the marriage bed.
Now some commentators have tried to distort the teaching of Fr. Lauer, as if he only condemned oral sex and other unnatural sexual acts when these acts were motivated by lust. But that is not the case. For he clearly states that oral sex ("oral-genital contact between spouses") is "not natural" and is "contrary to natural law." He considers oral sex to be just as immoral as "anal sex" or "homosexual activity". These are all unnatural sexual acts, and so they are each and all inherently immoral. He does not condemn these acts only when they are accompanied by the interior sin of lust, but in all cases. So while the "motivation behind oral sex is often lust," as unnatual acts, we have to conclude that these sexual acts are always immoral, as they are contrary to natural law. No orthodox Catholic teacher says that homosexual acts are only immoral when motivated by lust. So the same judgment applies to all other unnatural sexual acts. They are intrinsically immoral and never justifiable.
"May marriage be honorable in every way, and may the marriage bed be immaculate. For God will judge fornicators and adulterers." (Hebrews 13:4).
More on marital chastity in The Catholic Marriage Bed: A book of Roman Catholic moral theology.
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