What about childless heterosexual couples? How does an infertile heterosexual marriage differ from a same-sex marriage?
One big difference is that the heterosexual couple enjoys sexual complementarity, and the fullness that brings into their relationship, even if they cannot have children. Though the situation of an infertile couple is very different, there is a disturbing parallel to same-sex marriage in the situation of couples who simply choose not to have children.
Such couples are still able to have sex, the fullest physical expression of love between husband and wife. But they are doing something that profoundly disturbs the nature of the sexual act. Sexuality has two.aspects: the procreative (bringing forth children) and the unitive (strengthening the union of the couple). Artificially separating the unitive from the procreative brings discord to a marriage, distorts the relationship between husband and wife, and ends up harming their unity as spouses.
Pope John Paul II explains this with what he calls the “language of the body.” He observes that in the sexual act, man and woman implicitly give themselves totally to one another. That is what their bodies are saying, both symbolically and literally. Sexual expression, by its very nature, implies total gift of self to the other. The language of the body says, "I give myself to you completely, without reservation or condition."30 But sometimes that statement is a lie. Sometimes one or both do not give themselves completely to the other but instead use the other selfishly, as a pleasure object. Treating the other as an object is divisive rather than unitive.
There are several ways men and women can reduce one another to the status of object. Sex between couples who are not married and therefore do not bring a total commitment to their union are, in the Holy Father’s terminology, telling a lie with their bodies, because their bodies speak a language of total, unconditional, and permanent self-giving when in fact they are doing nothing of the sort.31 In that sense, their sexual expression becomes a lie, because it misrepresents their relationship. Regardless of their feelings for each other, their sexual expression promises more than it objectively delivers.
Fornication and adultery are not the only ways couples can lie with their bodies. Married couples who are committed to exclusive, permanent, and unconditional love may also tell a lie with their bodies when they separate the procreative aspect of sex from the unitive through contraception. Here, the failure to give oneself fully is more subtle but nonetheless real. Deliberately frustrating the procreative.aspect of a sexual act creates a condition that makes self-giving only partial and reduces the spouse, in some degree, to a pleasure object used for selfish purposes.
This does not mean that sex can be truly self-giving only during fertile parts of a woman’s cycle. The Church has never taught that couples must have as many children as possible. Rather, it means that interference with fertility both arises out of spousal selfishness and increases it. The Church approves natural family planning, in which couples abstain during fertile periods when they prayerfully have determined that there is a need to avoid pregnancy. In these cases the spouses are not separating the unitive and procreative.aspects of a sexual act; they are simply refraining from performing the act. Similarly, sex after menopause or when suffering from other forms of infertility do not divide the unitive from the procreative. The couple’s act is still ordered toward procreation; it is simply that procreation will not occur.32
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