Can morality change based on natural law?

Natural law is based on nature. But what if our understanding of nature changes? Does morality based on the underlying understanding change also? I can think of one way in which it did.

In the Old Law, it was forbidden to have sex during menstruation. This was listed as an abomination along with homosexuality, incest, and bestiality. It was unclean. The punishment was to be cut off from the people of God.

Since the New Law, there has been some variability in opinion about this practice. In the early Church and middle ages, it was generally regarded as sinful, though the sin would not be imputed to the woman if her husband demanded sexual intercourse during that time. It was considered sinful because in the middle ages there was the common belief that sex during menstruation caused miscarriages.

In the early modern period, it was demonstrated by science that sex during menstruation does not cause miscarriages. Subsequently, to my knowledge at least, having sex during that time by married couples is not considered sinful in the least degree.

In this case, it seems that a developing understanding of human nature has affected the moral conclusions drawn from that understanding, such that what was once considered a sin is no longer considered sinful.

Can this work in the opposite direction? Can something that was once considered not sinful later be considered sinful if our understanding of human nature develops? I was thinking about this today after having recently read about the concept of concealed ovulation.

Scientists have noted that in females of the human species ovulation is concealed. In most other animals, including those with whom we share recent common ancestors, ovulation is not concealed. In these species, a female's fertile period is conspicuously displayed, and in fact she can only engage in sexual intercourse during that period. Human females may have sex during any part of their cycle, and the timing of her ovulation is not obvious.

There is a diversity of opinion among scientists as to why humans evolved to have concealed ovulation. Some argue that it may confer survival advantages, while others say it may have just been a coincidence or an accident following from some other selective process.

Catholics, however, are not allowed to believe that the human being is a product of chance or blind selections made of random mutations. Catholics must believe that God guided the process, that the way we are is intentional (if the way we are by nature is not intentional, arguably we could not base morality on our nature). Therefore, we can rule out the possibility that concealed ovulation is an accident or coincidence. God intended human ovulation to be concealed.

Where am I going with this? When I was thinking about these things earlier, it seemed to me that this conflicts directly with Natural Family Planning. Does NFP deserve to be called natural? Will we eventually find out that it is contrary to nature?

The Church has obviously and definitively allowed and endorsed the use of NFP, at least for the past 150+ years, allowing married couples to limit sex to infertile periods for grave reasons (though as far as I know, She has not explained what these grave reasons are, nor have I ever heard anyone speaking on NFP emphasize this stipulation). But, could the Church change her position on this issue if there were developments in our understanding of human nature, for instance if we discovered that human ovulation is meant to be concealed by nature?

If ovulation is meant to be concealed, and the aim of NFP is to expose what our nature intended to conceal, then the methods of NFP could be thought of as an abuse of the human intellect, the intemperate acquiring of knowledge, and to act on such knowledge would be an act against nature.

We know that nature forbids some kinds of knowledge. For instance, carnal knowledge of a member of the same sex is against nature. It is therefore not a stretch to argue that nature could forbid other forms of knowledge dealing with our generative power.

Now, the Church has the definitive voice on morals, so I would not think that anyone should have their conscience disturbed on this matter unless the Church ever makes a statement about this. Still, it is something to think about on a speculative level.