The Magisterium teaches that to be moral, each and every knowingly chosen act must have three good fonts of morality:
- intention (the purpose or reason that the act was chosen)
- moral object
- circumstances
Any one or more bad fonts makes the act a sin. Good intentions are necessary but not sufficient. A correct choice of actions is also needed. Any act with an evil moral object is intrinsically evil, and therefore never justified by intention or circumstances.
In order to be moral, each and every sexual act must be marital, unitive, and procreative.
“The Church, nevertheless, in urging men to the observance of the precepts of the natural law, which it interprets by its constant doctrine, teaches that each and every marital act must of necessity retain its intrinsic relationship to the procreation of human life. This particular doctrine, often expounded by the magisterium of the Church, is based on the inseparable connection, established by God, which man on his own initiative may not break, between the unitive significance and the procreative significance which are both inherent to the marriage act.” (HV 11-12)
Certain types of sexual acts are inherently non-procreative; these are called unnatural sexual acts (oral, anal, or manipulative sexual acts). This type of act is intrinsically evil, and therefore never justified by intention (such as the intention to commit a natural sexual act at a later time), nor by circumstances (such as that the unnatural act occurs in the context of a natural act). The purpose of foreplay does not justify an intrinsically evil act.
“Each and every sexual act in a marriage needs to be open to the possibility of conceiving a child.” (USCCB Catechism, p. 409)
CCC 1756 “It is therefore an error to judge the morality of human acts by considering only the intention that inspires them or the circumstances (environment, social pressure, duress or emergency, etc.) which supply their context. There are acts which, in and of themselves, independently of circumstances and intentions, are always gravely illicit by reason of their object; such as blasphemy and perjury, murder and adultery. One may not do evil so that good may result from it.”
See this article on sexual ethics by theologian Alice von Hildebrand at the Catholic News Agency website:
catholicnewsagency.com/document.php?n=999