Besides appealing to Scripture and Tradition (Church teaching through the centuries), the Catholic approach to morality also relies heavily on human reasoning. The argument from natural law is a prime example. Other instances are the findings of the human sciences or the evidence of people’s personal experiences.
Arguments from natural law are inconclusive because the nature of human sexuality is debated. Procreation is certainly one aspect of sexuality. Yet the Catholic Church allows marriage between known sterile couples and sex between couples beyond child-bearing age. Moreover, Catholic teaching has recently emphasized the “unitive” aspect of sex—loving, caring, interpersonal sharing. Surely, the personal, not the biological, is the prime meaning of sex among human beings.
Similarly, the conclusion of the human sciences is that homosexuality is biologically based, a natural variation affecting about 10% of the population, often fixed by early childhood, and in no way pathological. (This 10% includes both the exclusively and the predominantly homosexual.) Likewise, while some may condemn practicing homosexuals as Godless and sinful, contemporary lesbian and gay Christians recognize their self-acceptance as a graced moment and report that, since coming out, they are happier, healthier, and closer to other people and to God.