You should invest in a good set of commentaries or two. I have Matthew Henry's in book form, Calvin's on CD-ROM (<--whew, that ages me!). I had Matthew Poole's once on disk (or is it disc?) but lost it.
Anyway, I've found that the Puritans and the Reformers are pretty much agreed on most things. The scholarship and devotion of both Henry and Calvin are unimpeachable.
Said all that to say this: to me, when Paul speaks of nature, he means natural law.
For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves: - Romans 2:14
And further down speaks of outward circumcision as unnatural
And shall not uncircumcision which is by nature, if it fulfil the law, judge thee, who by the letter and circumcision dost transgress the law? For he is not a Jew, which is one outwardly; neither is that circumcision, which is outward in the flesh: But he is a Jew, which is one inwardly; and circumcision is that of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God. - Romans 2:27-29
Calvin:
Doth not even nature itself He again sets forth nature as the mistress of decorum, and what was at that time in common use by universal consent and custom — even among the Greeks — he speaks of as being natural, for it was not always reckoned a disgrace for men to have long hair. 638 Historical records bear, that in all countries in ancient times, that is, in the first ages, men wore long hair. Hence also the poets, in speaking of the ancients, are accustomed to apply to them the common epithet of unshorn 639 It was not until a late period that barbers began to be employed at Rome — about the time of Africanus the elder. And at the time when Paul wrote these things, the practice of having the hair shorn had not yet come into use in the provinces of Gaul or in Germany. Nay more, it would have been reckoned an unseemly thing for men, no less than for women, to be shorn or shaven; but as in Greece it was reckoned all unbecoming thing for a man to allow his hair to grow long, so that those who did so were remarked as effeminate, he reckons as nature a custom that had come to be confirmed.
Henry
VI. He enforces his argument from the natural covering provided for the woman (v. 13-15): "Judge in yourselves—consult your own reason, hearken to what nature suggests—is it comely for a woman to pray to God uncovered? Should there not be a distinction kept up between the sexes in wearing their hair, since nature has made one? Is it not a distinction which nature has kept up among all civilized nations? The woman's hair is a natural covering; to wear it long is a glory to her; but for a man to have long hair, or cherish it, is a token of softness and effeminacy." Note, It should be our concern, especially in Christian and religious assemblies, to make no breach upon the rules of natural decency.
That law is that there needs to be a distinction in the grooming and attire of men and women. That's as far as the more civilized Gentiles could see it. Paul was saying, "Does not even nature teach you this very fact? Even the Gentiles can see this much." But he takes it further by explaining the grooming and attire should reflect God's created order in the sexes and their relationship to one another in the Lord.
When Paul said, "no such custom," most commentators are agreed that he said that the churches of God have no such custom in contending with societal norms that are in harmony with the laws of nature.
Here's a good online library:
https://ccel.org/
