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The conjugal act is natural sexual intercourse between a married heterosexual couple that's open to life and love. It's open to procreation and, therefore, to charity. Charity is a divinely infused virtue given to those who obey God's commands, which enables them to love unselfishly.
The conjugal act is more than mere sexual stimulation. It's called the marital act as it is occurs only within marriage and is a natural act meaning it's open to procreation. Frustrated sexual acts, including contraception and sodomy, aren't procreative nor unitive. They're acts of self-love.
Blessed Paul VI, in paragraph 12 of his 1968 encyclical Humanae Vitae, spoke of the "unitive significance and the procreative significance" that are "both inherent to the marriage act." He elaborated, "And if each of these essential qualities, the unitive and the procreative, is preserved, the use of marriage fully retains its sense of true mutual love and its ordination to the supreme responsibility of parenthood." Blocking the procreative or life-giving aspect of sex also blocks the unitive or love-giving aspect of sex as well.
Many people wrongly thought they could express love within marriage by contraceptive sex. This error spread from safe sex within marriage to same-sex relationships outside of marriage. Pope John Paul II dispelled this error in his 1984 commentary on HV. In paragraph six he writes, "[T]he conjugal act ... artificially deprived of its procreative capacity, ceases also to be an act of love."
The Catechism of the Catholic Church also equates the unitive aspect of the conjugal act with true love. Paragraph 2369 reads, "By safeguarding both these essential aspects, the unitive and the procreative, the conjugal act preserves in its fullness the sense of true mutual love."
The Church teaches in canon 1061 in the Code of Canon Law that only a conjugal act consummates a marriage. She further elaborates that contraceptive sex is not a conjugal or marital act and doesn't, therefore, consummate a marriage.
Study the mystery of married love in Church Militant's Premium show Remaining in the Truth—The Sacrament of Matrimony.