Are Diet Sodas Immoral?

In the last month, both Jeff and Claire have said something like "Wouldn't it be great if I could just eat forever and never get fat?" Thinking this was a kind of "teachable moment" I replied that since the purpose of food is for nourishment, then it would be contrary to the aim or end of food to be consumable without nourishing -- and of course that is what you would need to have food you could eat endlessly without getting fat. To desire such a thing is gluttony, a sin. (I'm not quite sure my reply was this well phrased, but that was the idea).

This got me to thinking: isn't diet soda then an exact parallel to contraception? In each case we have a pleasurable activity (eating, sex) which has a natural end (nourishment, procreation), and in which technology has created a way for us to enjoy the pleasure without making ourselves susceptible to the natural end? Augustine in his Confessions speaks of the pleasure in eating as something we must not seek as something good in itself, but accept only on account of the nourishment to be gotten from it. Of course, there is the difference that in sex, the natural end is the creation of a person who is an end in him or herself. But this only makes the comsumption of unnatural no-calories foods to be even less justifiable than that contraception, which can conceivably be justified on the basis of inability of the couple at the moment to raise a child in the proper way. Moreover, while the pleasure of sex has the natural end of bonding the couple who will be the parents of the child (nicely summarized as the "two B's, babies and bonding" purpose of sex), pleasure in food is related solely to nourishment and has no related purpose.

Is there a problem here? Is there something wrong in my reasoning? Should the consumption of sugar substitutes, olestra, and so on, be considered inherently immoral?