Objection 1. It would seem that no pleasure is not natural. For pleasure is to the emotions of the soul what repose is to bodies. But the appetite of a natural body does not repose save in a connatural place. Neither, therefore, can the repose of the animal appetite, which is pleasure, be elsewhere than in something connatural. Therefore no pleasure is non-natural.
Objection 2. Further, what is against nature is violent. But "whatever is violent causes grief" (Metaph. v, 5). Therefore nothing which is unnatural can give pleasure.
Objection 3. Further, the fact of being established in one's own nature, if perceived, gives rise to pleasure, as is evident from the Philosopher's definition quoted above (Article 1). But it is natural to every thing to be established in its nature; because natural movement tends to a natural end. Therefore every pleasure is natural.
On the contrary, The Philosopher says (Ethic. vii, 5,6) that some things are pleasant "not from nature but from disease."
I answer that, We speak of that as being natural, which is in accord with nature, as stated in Phys. ii, 1. Now, in man, nature can be taken in two ways. First, inasmuch as intellect and reason is the principal part of man's nature, since in respect thereof he has his own specific nature. And in this sense, those pleasures may be called natural to man, which are derived from things pertaining to man in respect of his reason: for instance, it is natural to man to take pleasure in contemplating the truth and in doing works of virtue. Secondly, nature in man may be taken as contrasted with reason, and as denoting that which is common to man and other animals, especially that part of man which does not obey reason. And in this sense, that which pertains to the preservation of the body, either as regards the individual, as food, drink, sleep, and the like, or as regards the species, as sexual intercourse, are said to afford man natural pleasure. Under each kind of pleasures, we find some that are "not natural" speaking absolutely, and yet "connatural" in some respect. For it happens in an individual that some one of the natural principles of the species is corrupted, so that something which is contrary to the specific nature, becomes accidentally natural to this individual: thus it is natural to this hot water to give heat. Consequently it happens that something which is not natural to man, either in regard to reason, or in regard to the preservation of the body, becomes connatural to this individual man, on account of there being some corruption of nature in him. And this corruption may be either on the part of the body--from some ailment; thus to a man suffering from fever, sweet things seem bitter, and vice versa--or from an evil temperament; thus some take pleasure in eating earth and coals and the like; or on the part of the soul; thus from custom some take pleasure in cannibalism or in the unnatural intercourse of man and beast, or other such things, which are not in accord with human nature.
This suffices for the answers to the objections.
Source: Summa Theologica (New Advent)