In Christian theology, the sexual act is an icon of the Trinity and of the saving relationship between Jesus and the Church.
It is an icon of the Trinity because God is a communion of persons, who all completely give of their entire selves to each other. The person of the Father subsumes his entire existential being and substance to beget the Son, who, composed of the same divine essence, gives back the same total self-giving love in turn to sustain the Father. This total self giving of two divine persons begets the holy Spirit, who is interpreted to be the love between the Father and Son that is shared between them.
When a man and woman marry each other, they ideally pledge their entire lives, wills, and bodies for the good of each other. Sexual intercourse unites the man and woman in their love, and through their love, they beget another person- a child- much like the love between the Father and Son begets the holy Spirit. When the procreative aspect of sex is willfully removed by the man and women, the act no longer serves as an image of the divine love because the creative power of the man and women have been withheld from each other. They haven’t given their whole selves to the other.
Sex is an icon of the saving relationship between Jesus and humanity in the sense that Jesus is the groom and humanity (the Church) is his bride. Jesus (the man) sacrificed his life and body for the Church (the woman), so that she might have new life within her (a child). Any form of sex that removes the procreative aspect would be like if Jesus chose to prevent his bodily sufferings- to withhold his body so that we would have no new life within us.
For the Christian, sex is about the affirmation of the goodness of persons- of the goodness of human life. The man sees the body of the woman, and his sexual arousal tells him that her body and her existence is a good thing. Likewise in turn for the woman. Their act of sexual intercourse, which is an affirmation of the goodness of each others’ existence, is capable of producing a new personal life (a child). Their affirmation of the goodness of human life actually creates another human life!! To deliberately withhold their fertility from each other through contraception changes the nature of the act, so that the act no longer says that human life is good, but that human life is something to be avoided. The act of sex becomes contradictory. Additionally, it is avoiding giving the total self to the other, which was promised in the wedding vows. In short, the sexual act no longer reflects the divine love which it was created in image of.
So any use of the sexual faculties that is done without the context of free (not rape or compulsory), total (no withholding fertility), and faithful (no adultery) love, is denying the objective character that God created it with. Contraception is wrong because it withholds a part of each person from each other during the sexual act, and it contradicts the intrinsic meaning of the sexual act (affirmation of the goodness of human life). Extramarital sex is wrong because it is not done within the context of a total pledge of one’s life to the other (withholding a part of each person from the other). Masturbation is wrong because it is not done within the context of actually loving another person.
In short, if the sexual action of the person does not reflect the character of divine love, then it is immoral.