I've often wrestled with the question of what I would be forced to believe if science ever discovered a "gay gene." With the mapping of the human genome, Francis Collins has affirmed that this is not a remote possibility. Would this mean that, since one can be genetically predisposed to be gay, that the homosexual inclination is in conformity with the natural law?
I think that part of this objection has to do with too closely conflating natural law theory with biological determinism. No single genetic inclination makes that inclination "human." After all, the evolutionary development of the human being is full of mistakes and wrong terms, and so it can only be suspected that our genetic make-up is full of deviations from what it means to authentically be human. If anything, evolutionary theory has helped us to understand precisely how much we are not dependent upon our genetic make-up, or at least that we are capable of rising above. Due to the evolutionary process, some people are genetically more prone to aggression than others, but this does not make aggression that leads to murder right.
Natural law reflects on what it means to be human as body and soul. Therefore it includes the body, but not in a deterministic way. With existentialist philosophy, it agrees that what primarily constitutes human nature is precisely freedom -- not freedom in the abstract, but human physical freedom for, not from. What it means to be human according to natural law theory is to be an embodied person who exercises freedom as a tool towards the accomplishment of the single vocation of the human race. This ultimate vocation is union in God. Because our human nature is freedom for, we must eliminate all practices -- even negative genetic holdovers from evolution that can be compounded by selfishness -- that, as I have argued before, are not self-diffusive of the person, which is the inherent structure of authentic freedom. Homosexual behavior is one such practice.
Nathan O'Halloran, SJ