A sexual act is any deliberate use of the genital sexual faculty.
Sexual acts are not exempt from the moral law. To be moral, each and every knowingly chosen sexual act must have three good fonts of morality. The intention must be good, the moral object must be good, and the good consequences must outweigh any bad consequences.
In order to have a good moral object, each and every sexual act must be marital and unitive and procreative. Each and every moral sexual act always has these three meanings: marital, unitive, procreative. The deprivation of any one or more of these meanings from the moral object causes the sexual act to be intrinsically evil and always gravely immoral.
The natural sexual act is genital-to-genital intercourse between a man and a woman. This act is unitive and procreative. Natural sexual intercourse between a husband and wife is called natural marital relations. Only natural marital relations is martial and unitive and procreative.
The use of contraception deprives the act of natural intercourse of the procreative meaning, causing the sexual act to be non-procreative. The use of contraception is intrinsically evil and always gravely immoral because it deprives sexual relations of its procreative meaning, which is required by God for sexual acts to be moral. Therefore, natural marital relations must always be open to life (not contracepted).
A non-marital sexual act is any type of sexual act outside of marriage. Acts of adultery, pre-marital sex, and masturbation are non-marital. All non-marital sexual acts are intrinsically evil and always gravely immoral because these acts lack the marital meaning, which is required by God for sexual acts to be moral.
An unnatural sexual act is any type of sexual act that is not unitive and procreative. Examples of unnatural sexual acts include oral sexual acts, anal sexual acts, and manipulative sexual acts (i.e. masturbation of self or of another). All unnatural sexual acts are intrinsically evil and always gravely immoral because these acts lack the unitive and procreative meanings, which are required by God for sexual acts to be moral. These acts are not procreative because they are not the type of act that is inherently directed at procreation. These acts are not truly unitive, even if there is a certain mere physical union of body parts, because this is not the type of sexual union intended by God for human persons. Unnatural sexual acts are not justified by being done within marriage because the moral law requires each and every sexual act to be not only marital, but also unitive and procreative.
To be moral, each and every sexual act must be marital and unitive and procreative. All non-marital sexual acts, all non-unitive sexual acts, and all non-procreative sexual acts are intrinsically evil and always gravely immoral. All such acts have an evil moral object, and so they are not justified by intention, or by circumstances, or by other acts.