Natural law theory is a rational argument based on the existence of certain  undoubtable truths, such as "Do the Good." Deductions from that maxim lead to a moral conclusion. In the case of LGBT, the deduction has climbed down several steps. 

The truth of the conclusion depends on: 1) The validity of the deductive argument. But it could be logically flawed. 2) The facts. But new facts might arise.  3) The adequacy of the perspective used. But a better perspective might appear. 

Catholic moral argument is usually based on an Aristotelian hypothesis called an  "unchangeable substance." It underpins every reality but cannot be directly seen, an explanation of continuity in the midst of change. An invisible underlying substance decrees our sexual behavior, and it must be respected, or the act is "unnatural." Taught by Fr. Don Gelpi, S.J., I have moved to a philosophy which questions logic, accepts new facts, and adopts an historical perspective based on shared systematic inquiry.