I’m not an expert on natural law, but I think we can draw some conclusions here to help with the progression of this discussion. Pithy remarks and witticisms are all fun, but this is a legitimate issue that should have some concrete and objective waypoints along the way. We need to recognize that we are dealing with human persons (created by God) and this is a real issue. Those with SSA need legitimate dialogue, encouragement, and prayers.
We all have our crosses to bear. It isn’t about fairness as to who gets what cross. We just have these crosses. We can either bear them up, or let them drop. The only thing unfair is to not be willing to bear our crosses for the love of Jesus Christ.
All that being said, let’s talk about what Natural Law is. For much of this I am going to use NewAdvent as a source. Thomas Aquinas says that Natural Law is “nothing else than the rational creature’s participation in the eternal law.” (I-II, 94.) Well, what then is Eternal Law? NewAdvent goes on to say that the Eternal Law is God’s wisdom. I don’t think any of us will deny that there is a definitive created order to the cosmo. On the physical plane, we have seasons, physical laws/theories, cycles of law and death, laws of harmony—order. There is an order to our lives as we exist in Time. On the metaphysical plane, we know there to be an order to morality, ethics, theology, philosophy, etc. All things are signifiers of something else.
All things point towards a certain end. If things are created, they are created for a purpose. Sometimes they are created for the sole purpose of being loved. Sometimes they have an actual, utilitarian purpose. Hammers, beds, chairs, cars are created for a useful purpose. But what about man? What is man created for? Well, God intended man for love. Thus, the justification of many to say that SSA is justified by a shared love between a man and a man. There is no question that there can be a legitimate love between two people of the same sex. As Aquinas says, “Love is willing the good for another.” Will you deny the ability of a man to will the good for another man? Absolutely not. However, the question to ask is, is this love between those of SSA the true and complete form of Love? That is wherein the difficulty lies.
God created man for love. God loves man, and desires the love of man. What is the truest form of love? The complete giving of self, body and spirit, to another. Christ gave His spirit and His body for man. Every time we approach the sanctuary, we take Christ into ourselves and are united. One of the tenets of marriage is the willingness and desire to procreate and have children. Why is this? It is the procreative act that we are acting most similiarly to God. Husband and Wife become One to procreate. All of creation came from a Single Being (Albeit 3 persons in one God). It is the highest form of Love to become One, and to issue forth new life.
I’ll try to bring this back to Natural Law. As Aquinas said, it is the rational being partaking in the eternal law. Eternal Law is the wisdom of God; God created and so ordained the cosmos in an objective order. Man participates in that order with his reason (Reason is used for the discernment of truth, and thus, a discovery of that order). Man exists to love God, and so must direct all of his conscious actions towards loving the Creator. In order to love the Creator to the height of possibility, he must give himself body and soul to another. Through this union, the Creator is loved and honored and so new life comes into the world.
I think what I am describing is the usual case–and this is taking into consideration that all biological specifications are met (discounting barreness, sterility, age, etc.). This is also not considering the Priesthood, which is altogether a different situation. Man was meant for physical union, and the only way this is not to be the case is with the aid of Grace.
Can a homosexual couple come together and procreate? No, they cannot. This doesn’t discount the veracity of their love, but it prohibits the fulfillment of the love. Man was designed for woman for the giving of self, not just for the emotional attachment and willfull love for another. The consequence is just as important as the feeling. Both are necessary.