It’s easy to get confused about what the word “natural” means because it can and has been used in a lot of different ways. A lot of the times it is used like, “Plants are natural and Glasses are artificial.” In this context, what is “natural” is that which is found in the physical realm before humans arrived on the scene.
What Aquinas mean by natural (or the relevant sense of the word) is something like, “That which corresponds to its form and/or actualizes its final cause” (end goal or telos). So Aquinas doesn’t use the word “natural” in the same sense as one who fallaciously appeals to nature. An example of the fallacy would be:
1. Glasses or Shoes are artificial and not “natural”
2. Therefore, it’s wrong to wear them
Aquinas’ model of natural law ethics doesn’t say this at all. In fact, Aquinas would encourage wearing shoes and glasses because they help the organism fulfill its final goal of “seeing” (for the eyes) or “walking” (for the feet). Similarly, Aquinas would have no problem with land bridges, cars, or modern day medicine. All of these things help humans in reaching their goals, even though they aren’t “natural”. But again, this use of the word “natural” doesn’t concern Aquinas in the least.
For Aquinas things like homosexuality would be wrong because the person is frustrating the final (biological) goal of their private parts. Note: Frustrating the final end of something is not the same thing as abstaining from said action on Aquinas’ model.
Also, eating too much food everyday is wrong because it is frustrating the end goal of being a healthy human being. Likewise, murder is frustrating the end goal of societal peace.