Is there an intrinsic morally significant difference between having sex solely for one's pleasure and having sex solely for money? To sharpen the question, let's ask: Is there a morally significant difference between having sex solely for one's pleasure and having sex for money which one intends to use solely as a means to one's pleasure?
Both are cases where the sex is engaged in solely for hedonistic ends, but in the one case the pleasure is achieved more indirectly. Still, in both cases there is some indirectness. Sex, in and of itself, need not be pleasurable. In both cases, it seems to be engaged in as a mere means to pleasure. The difference, however, is that in the one case, the pleasure is the pleasure of this very sexual act, while in the other case, the pleasure is a different pleasure (e.g., the pleasure of driving a nice car, or the pleasure of sex with someone else whom one wishes to seduce in an expensive way). So, in the case of sex solely for one's pleasure, the pleasure is more closely tied to the sex, and it may even be a mistake to talk of the pleasure as a distinct end. If so, then there is a significant—and perhaps morally so—difference between the two cases: in the money case, sex is engaged in purely instrumentally, while in the pleasure-of-sex case, the end is too close to the sex to call the sex purely instrumental.
This distinction, however, imports into the original question something that wasn't there. Granted, there is a difference between sex solely for the pleasure of the sex, and sex for the sake of money which one wants for the sake of some other pleasure. But in the case I originally specified, I did not suppose a case of sex for the sake of the pleasure of the sex; I supposed a case of sex for the sake of pleasure simpliciter. And when one's end is pleasure simpliciter, then one's action plan involves a fungibility of means: one looks around for the ways to get a lot of pleasure, considering whether it is more convenient to get pleasure by proving a new theorem, or having sex, or eating cake, or volunteering at a shelter—or by having sex for money and then using the money to buy a pleasure. Insofar as one is having sex solely for the sake of pleasure, one is prima facie indifferent between these options except insofar as they produce different levels of pleasure with different degrees of convenience.
And if so, then it does not seem that there is a significant moral difference between sex solely for money solely for pleasure and sex directly solely for pleasure. In particular, it follows that if we think that non-marital sex for money is always wrong, we will conclude that non-marital sex solely for pleasure is always wrong; and if we think that marital sex for money is always wrong, we will likewise conclude that marital sex solely for pleasure is always wrong. (To be honest, I think some cases of marital sex for money—say, when one is starving and one's spouse refuses to provide food except on condition of sex—are more defensible than marital sex solely for pleasure.)
However, there is a difference between sex for the pleasure of the sex and sex for money. Sex for the pleasure of the sex is not solely hedonistic. The hedonist as such does not care what she is taking pleasure in, convenience, consequences and intensity being kept constant. Insofar as one cares about what one is taking pleasure in, one is not a pure hedonist. In a case of sex for the pleasure of the sex, the sex is not present purely instrumentally. Here, we also should distinguish sex for the pleasure of sex from sex for the pleasure of the sex. The pleasure of sex can be achieved apart from sex, say by direct neural input. The pleasure of the sex can only be achieved through the sex. It may be that there is little moral difference between sex for the sake of the pleasure of sex and sex for the sake of money. After all, one could have sex for the sake of money in order to get the pleasure of sex—perhaps one is saving up for a neural sexual pleasure implant. But there is a moral difference between sex for the sake of the pleasure of the sex and sex for the sake of money. Say that a sophisticated hedonist is someone for whom not only the intensity, convenience and consequences of a pleasure matters, but the kind of pleasure also matters. Maybe the sophisticated hedonist wants to have a variety of kinds of pleasure, or maybe she has arbitrarily chosen some pleasures over others. In any case, the person who has sex for the sake of the pleasure of the sex is neither a pure hedonist nor a sophisticated hedonist, for she not only cares for the kind of pleasure, but also that which it is had in.
One is unlikely—perhaps it is even an impossibility—to value the pleasure of the sex without non-instrumentally valuing the sex. There may well be people who have sex solely for pleasure. For instance, if Sally wants to have some pleasure and goes through all the options and chooses the one with the best balance of intensity and convenience, and that happens to be sex, she may be having sex solely for pleasure. But such cases are, I think, rare. The pure case of someone who wants to have sex for the sake of the pleasure of sex is less rare. Such a case would require the person to be indifferent as to the gender, age, appearance and species of the sexual partner, except insofar as this impacts convenience, consequences and the pleasure received. Maybe some people do have such an indifference—the only reason, for instance, why they prefer their partners to be of their own species is that they find bestiality to lack something of pleasure.
There is a further kind of distinction we should draw at this point, a distinction between having sex for the sake of the pleasure of the sex—of this particular sexual act with this person at this time—and having sex for the sake of the pleasure of this sort of sex. Thus, the person who cares about the appearance of their sexual partner (typically) seeks the pleasure of sex with a good-looking person. The number of people who have sex for the sake of pleasure is probably small, the number of people who have sex for the sake of the pleasure of sex is probably also small, but the number of people who have sex for the sake of the pleasure of sex of a certain sort (where the sort is either specified by specifying the kind of sexual act or the kind of partner or both) is probably larger. And this, too, I think is not very different morally from sex for money. After all, one might well be having sex for money for the sake of the pleasure of sex of the preferred sort. And I think this is morally objectionable, and apt to make an object of the partner.
On the other hand, sex for the sake of the pleasure of the sex requires or at least tends to require valuing the sex with this person at this time non-instrumentally (actions are individuated by agent, time, patient, etc.) And that's different.
The Christian tradition has unanimously condemned sex for the sake of one's own pleasure. But it is not clear that this condemnation of hedonistic sex applies in the case of sex for the sake of the pleasure of the sex—for in that case, the intended end, "the pleasure of the sex", is partly constituted by the sex itself, and hence the sex is engaged in for the sake of the pleasant good of the sex, rather than for the sake of pleasure alone. And if the sex is intrinsically unitive, this may well be sex for the sake of pleasant union, which is a species of sex for the sake of union, which in turn is taken to be permissible by the tradition. Of course, this is very speculative, and the tradition is authoritative while my interpretation of it is not.