I'm going a bit off target of my G rating today, but what I've written reflects Church teaching on morality without specific graphic detail. If you are a younger reader, please have your parents read and approve the following post before continuing to read for yourself.
Among the many teachings on marriage, the Catechism of the Catholic Church has the following to say about the nature of married love:
2363 The spouses' union achieves the twofold end of marriage: the good of the spouses themselves and the transmission of life. These two meanings or values of marriage cannot be separated without altering the couple's spiritual life and compromising the goods of marriage and the future of the family.
The conjugal love of man and woman thus stands under the twofold obligation of fidelity and fecundity.
2364 The married couple forms "the intimate partnership of life and love established by the Creator and governed by his laws; it is rooted in the conjugal covenant, that is, in their irrevocable personal consent." Both give themselves definitively and totally to one another. They are no longer two; from now on they form one flesh. The covenant they freely contracted imposes on the spouses the obligation to preserve it as unique and indissoluble. "What therefore God has joined together, let not man put asunder."
In other words, the purpose of marital love has two aspects: the loving, unitive bond between husband and wife and the possibility of participating in the creation of new life. If those two aspects of conjugal love are separated then the marriage is compromised--missing something important or integral, so to speak. Secondly, marriage itself creates a bond between husband and wife that is established by God and cannot be broken.
As an NFP teacher I love this teaching. It makes so much sense and it is incredibly consistent and true. It is this Truth that makes self-gratification sinful and so damaging to souls, because it removes the loving union between spouses. It is this Truth that makes out of wedlock relations sinful and so damaging to souls, because relations outside of marriage lack that unitive bond Blessed by God. It is this Truth that makes homosexual unions sinful and so damaging to souls, because those types of unions, by their very nature, cannot contribute to the creation of new life and lack also the Blessed recognition by God. It is this Truth that makes infidelity sinful and so damaging to souls, because once again the relationship occurs outside of marriage and destroys the unitive bond between husband and wife. It is this Truth that makes contraception sinful and damaging to souls, because by definition contraception prevents the ability to transmit new life. Finally, it is this Truth that makes artificial insemination and fertilization sinful and damaging to souls, because these fertility techniques remove the unitive aspect of the marriage act. It is the same teaching, the same Truth that applies to each of these situations. For some reason I find that very reassuring.
Often when seen as separate teachings, Truths can be hard to understand and agree with, but when viewed springing from one single Truth, hard teachings do come to make sense through their applied consistency.