This categorization of homosexual behavior as "contrary to nature", (i.e. “unnatural,”) was adopted with particular vehemence by Hellenistic Jewish writers, who tended to see a correspondence between the philosophical appeal to "nature" and the teachings of the Law of Moses, "The Law recognizes no sexual connections," writes Josephus, "except for the natural (kata physin) union of man and wife, and that only for the procreation of children. But it abhors the intercourse of males with males, and punishes any who undertake such a thing with death."' In Paul's time, the categorization of homosexual practices as para physin was a commonplace feature of polemical attacks against such behavior, particularly in the world of Hellenistic Judaism. When this idea turns up in Romans 1 (in a form relatively restrained by comparison to the statements of some of Paul's contemporaries, both pagan and Jewish), we must recognize that Paul is hardly making an original contribution to theological thought on the subject; he speaks out of a Hellenistic-Jewish cultural context in which homosexuality is regarded as an abomination, and he assumes that his readers will share his negative judgment of it. In fact, the whole design and logic of his argument demands such an assumption. Though he offers no explicit reflection on the concept of "nature,", (i.e. “natural,”) it appears that in this passage Paul identifies "nature" with the created order.

Richard B. Hays,The Moral Vision of the New Testament