The reason why people of the same sex who love each other and are willing to be faithful for life cannot get married is because there is more to marriage than love and faithfulness. These are necessary ingredients, but they are not the only ones. Our modern culture finds it difficult to understand why marriage is only for heterosexuals because it does not understand the purpose and meaning of marriage and sexuality.
This difficulty exists partly because contraception has divorced procreation from sex. If a heterosexual couple can engage in sexual activity that is opposed to the transmission of life, why can’t members of the same sex do likewise?
To understand why marriage is only for the joining of a man and a woman, we need to step back from the influences of the world and define the essential characteristics of marriage and sex.
We did not invent either one. They are God’s creation, and according to his designs, when a man and woman make love, they are saying their wedding vows with their bodies. Such a concept is easy to understand when you consider the essence of marriage. For a valid marriage to take place, the union must be free, total, faithful, and designed to give life. All these characteristics are necessary. For example, imagine a couple who agree to marry and have children but refuse to be faithful to one another. This is not a marriage. Likewise, if a couple are willing to have an exclusive and lifelong relationship but are incapable of having the sexual relations designed to give life, they would be incapable of marriage.
Since their bodies cannot express the vows of a married couple, they cannot get married. Because of this, some argue that the Church is “discriminating against gays.” But realize that the Church also does not allow impotent couples to marry. Not to be confused with sterility (where a couple is able to have intercourse but unable to have kids), impotency is where a person is incapable of having intercourse. Ever since the beginning sex has been an integral part of marriage. In Genesis we read, “A man leaves his father and his mother and clings to his wife, and they become one flesh” (Gen. 2:24).
Just as an impotent couple is unable to marry because they cannot become one flesh, one reason why members of the same sex are incapable of marriage is because they are incapable of intercourse. Just because two people are engaging in some form of sexual activity, this does not make them one flesh. This inability of the bodies to unite in a sexual way expresses the deeper reality that they were not meant to give themselves to each other in marriage. Therefore the Church has no authority to marry a couple who cannot become one. This is not easily understood by a culture that separates sex from marriage. Not surprisingly, the culture that first demanded sex without marriage now demands marriage without sex.
Although same-sex temptations, like temptations in general, are not in and of themselves sinful, Scripture and Church teaching condemn homosexual actions (Rom. 1:24–27; Gen. 19:1–29; 1 Tim. 1:8–10; CCC 2357–2359). Therefore, other Christians should lovingly encourage them to practice chastity, and not give in to the widespread popular notion that the real Christian attitude is "tolerance." Jesus does not call us to merely tolerate, but to love. St. Basil the Great once said, "[T]hose who pretend to be tolerant because they wish to flatter--those who thus fail to correct sinners--actually cause them to suffer supreme loss and plot the destruction of that life which is their true life." The true life of all people is life in God, and anything less that that will not bring them the joy for which they have been created.
If two members of the same sex are mutually attracted and they love one another, they will do what is best for each other. They desire union because of their love, but love desires more than a temporary physical union; it desires the good of the other. It desires heaven for the other person and will encourage him or her to embrace the virtue of chastity.