According to the Catholic Church, are sex positions that can’t bear children sinful?
The short answer is yes and no. That will depend on the circumstances involved.
Please bare with me here and I would like to think we are all adults here.
Before going on I would like to make one point absolutely clear: Since it’s the male orgasm that is inherently linked with the possibility of new life, the husband must never intentionally ejaculate outside of his wife’s vagina.
Remember what happened to Onan!
3 And she conceived, and bore a son, and called his name Her.
4 And conceiving again, she bore a son, and called him Onan.
5 She bore also a third: whom she called Sela. After whose birth, she ceased to bear any more.
6 And Juda took a wife for Her his firstborn, whose name was Thamar.
7 And Her, the firstborn of Juda, was wicked in the sight of the Lord: and was slain by him.
8 Juda, therefore said to Onan his son: Go in to thy brother's wife and marry her, that thou mayst raise seed to thy brother.
9 He knowing that the children should not be his, when he went in to his brother's wife, spilled his seed upon the ground, lest children should be born in his brother's name.
10 And therefore the Lord slew him, because he did a detestable thing. - Genesis 38:3-10
Even John Calvin wrote that "the voluntary spilling of semen outside of intercourse between a man and a woman is a monstrous thing. Deliberately to withdraw from coitus in order that semen may fall on the ground is double monstrous." Involuntary spilling of one’s semen as in a premature ejaculation would not be sinful in the intention was made to have natural sexual relations with one’s wife.
There was a time, that the only position the Church encouraged was the missionary position. We get this name, that is right from Catholic missionaries, who taught this was the most acceptable position for couples to have sex and had a greater chance of pregnancy, when in the lands of evangelization, especially during the 18th and 19th centuries.
Generally speaking example 1 would be considered grievously sinful. Generally speaking example 2 would be considered grievously sinful too. Example 2 may have possible exemptions.
Since example 3 and 4, the man still deposits his sperm into the vagina of his wife, it is not sinful. The openness to the procreated aspect of marriage remains intact. This is brought out in Geremia’s answer.
Are there possible circumstances in which oral or manual sex could be permitted?
In order to explain what the Church teaches about oral sex, one must first be aware of the Church’s teachings on the nature and purpose of all sexual expression.
First and foremost, the Church reserves all sex, including oral sex, for marriage. This isn’t to restrict our natural sexual impulses, but rather to save them for what they were properly intended, namely for procreation of children and to build unity between husband and wife. Pope Benedict spoke openly of his concern that limiting the Church’s attention on sex to “just moral prohibitions” can lead people to “have the impression that the church’s real function is only to condemn and restrict life. Perhaps too much has been said and too often in this direction—without the necessary connection to truth and love.”
While the words “oral sex” do not appear in the Catechism of the Catholic Church, the Church draws directives from its traditional teaching on sexuality to provide guidance. Many people are surprised to hear that even within marriage, the Church makes a distinction between oral “sex” and oral stimulation. If we define oral sex as orally stimulating the male partner to orgasm, then the Church prohibits that even for married couples.
Two books that offer specific insights into the Catholic Church’s teaching on oral sex are Christopher West’s ”Good News About Sex and Marriage: Answers to Your Honest Questions About Catholic Teaching” and Vincent Genovesi’s “In Pursuit of Love: Catholic Morality and Human Sexuality.”
West has sought to make Saint Pope John Paul II’s “Theology of the Body” accessible for a wider audience. He’s written several books and articles on the subject, and in “Good News About Sex,” which is a practical summary of this theology, West offers some instances in which oral stimulation (stimulating genitals but not to the point of ejaculation) is perhaps acceptable within marriage:
Foreplay: If the act of foreplay leads to sexual intercourse where the male climaxes into the female, then oral stimulation is certainly permissible for a couple to engage in within marriage.
Orgasm: If a man was able to orgasm during sexual intercourse but his wife did not, he may bring his wife to orgasm after intercourse in whatever way he chooses (manual or oral stimulation). The reverse, however, is prohibited. A man’s orgasm is always tied to his fertility, so, therefore, the Church states that oral sex that would end with a male orgasm outside of sexual intercourse is not permissible. West writes, “Since it’s the male orgasm that’s inherently linked with the possibility of new life, the husband must never intentionally ejaculate outside of his wife’s vagina. Since the female orgasm, however, isn’t necessarily linked to the possibility of conception, so long as it takes place within the overall context of an act of intercourse, it need not, morally speaking, be during actual penetration.”
No substitutions, please: Oral sex or stimulation can never be used as a replacement for sexual intercourse, but oral stimulation can be used to lead a couple to vaginal intercourse. Pope Benedict also points couples toward discovering love within sex instead of settling for substitutions for the real thing, stating: “No mechanical technique can substitute the act of love that two married people exchange as a sign of a greater mystery.”
Intimacy over arousal: Not every single sexual act, per se, need be procreative, but during the sexual act, there needs to be openness to procreative activity. So, there can certainly be oral stimulation throughout sexual activity within marriage, but if one is using oral sex simply to avoid pregnancy yet achieve orgasms, then one is limiting their sexual union to merely give arousal (sexual stimulation) rather than real intimacy (seeing and being seen for who you are).
Premature ejaculation: For something to be sinful, there needs to be both intent and full knowledge of that intention to do evil. If one were to orgasm prematurely (i.e., accidentally) that is not a sinful act. One needs to be mindful of their intention to sin.
The Church teaches that sex within marriage should be a loving expression of unity and openness to procreation. As Pope Francis explains in his apostolic exhortation “Amoris Laetitia,” desire and passion are part of the human experience, and those who enter into marriage together will continue to learn and grow in intimacy together over a lifetime. - What Does the Church Teach About Oral Sex?