Some Catholic couples mistakenly believe that, within marriage, a husband and wife can make use of any kind of sexual acts with one another. On the contrary, certain kinds of sexual acts are intrinsically disordered and always gravely immoral. Such acts cannot be justified in any circumstance, for any reason, regardless of intention, even within marriage.
Certain kinds of sexual acts are intrinsically evil and are therefore always immoral, regardless of circumstances, intention, or purpose.
Examples of intrinsically disordered sexual acts include: masturbation, homosexual acts, any sexual acts with more than two participants, oral sex, anal sex, manual sex, sexual acts involving objects or devices, etc.
These sexual acts can never be justified regardless of circumstances, intention, or purpose. These sexual acts are unnatural because they violate the natural law. The human person was designed by God so that sexual relations would consist in acts of genital-to-genital intercourse, open to life, between one man and one woman. Other kinds of sexual acts are contrary to this intention and purpose of God, which He designed within human nature.
So, for example, a husband cannot deliberately stimulate the genital organs of his wife in order to give her sexual pleasure, for such an action is defined within the Catechism as a type of sexual act which is "intrinsically and gravely disordered." The masturbation of another person is no less immoral than the masturbation of oneself. And regardless of whether this "deliberate stimulation of the genital organs" is done with the hand or the mouth or an object, it remains essentially the same kind of act, one which is intrinsically and gravely disordered, according to the Catechism.
Sexual Sins within Marriage