That is correct. But in the context of homosexual acts (the reason I brought up the topic of Natural Law) I would argue that they are clearly contrary to the natural law as they violate the biological, procreative purpose and design of the sexual faculty. So although our understanding can only ever be incomplete of the natural law in its totality there are certain areas where we can be certain of the immorality of a certain action.
Homosexual acts do not 'violate' a 'purpose'. They have their own purpose. Homosexual activities can easily be good or can easily be wrong, but it's incident-specific.
You're also making a fallacy by insisting upon a design and a purpose for that design.
Physical laws do not have those traits. Nuclear fission doesn't occur
in order to cause cancer. Gravity does pull
in order to suck apples to the ground.
Or, at least, there's no reason to suggest so (and especially no reason to insist so).
You're now shoehorning natural law into a conception of divine law. This, again, can cause people to become confused when trying to determine natural law. Natural law is not divine law, there's no logical reason for it to be, excepting a limited definition of 'divine'.
Homosexual acts have a wreath of purposes. There's a vast array of biological, emotional, and physical results. Yes, procreation is not one of them. Clearly, the 'purpose' of a homosexual act is not procreative, so it's not violating any procreative purpose.
What we see is scripture fortifying homophobic urges, or even worse, causing them.