When theologians and moralists speak of homosexuality, contraception, abor- tion, and other forms of human behavior as being unnatural and say that for that reason such behavior must be considered to be wrong, in what sense are they using the word unnatural? Are they saying that homosexual behavior and the use of contraceptives are  contrary to the scientific laws of nature, are they saying that they are  artificial forms of behavior, or are they using the terms natural and unnatural in some third sense?
They cannot mean that homosexual behavior (to stick to the subject presently under discussion) violates the laws of nature in the first sense [in- cluding, for example, Boyle’s law that the volume of a gas varies inversely with the pressure that is applied to it], for . . . in that sense it is impossible to violate the laws of nature. Those laws, being merely descriptive of what actu- ally does happen, would have to include homosexual behavior if such behav- ior does actually take place. . . .
If those who say that homosexual behavior is unnatural are using the term unnatural in the second sense as artificial, it is difficult to understand their ob- jection. That which is artificial is often far better than what is natural. . . . [Moreover,] homosexual behavior can hardly be considered unnatural in this sense. There is nothing artificial about such behavior. On the contrary, it is quite natural, in this sense, to those who engage in it. And, even if it were not, this is not in itself a ground for condemning it.
It would seem, then, that those who condemn homosexuality as an unnatural form of behavior must mean something else by the word unnatural, something not covered by either of the preceding definitions. A third possibility is this:
3. Anything uncommon or abnormal is unnatural. If this is what is meant by those who condemn homosexuality on the ground that it is unnatural, it is quite obvious that their condemnation cannot be accepted without further argument. The fact that a given form of behavior is uncommon provides no justification for condemning it. . . . Great artists, poets, musicians, and scien- tists are uncommon in this sense; but clearly the world is better off for having them, and it would be absurd to condemn them or their activities for their failure to be common and normal. If homosexual behavior is wrong, then, it must be for some reason other than its unnaturalness in this sense of the word.