Usually, it is often believed that desires for food and sex are based on natural (biological) needs. Epicurus too call them "natural" but claims that the fulfillment of the desire for food is "necessary", while the fulfillment of the desire for sex is "unnecessary". What does he really mean here? Is he wrong in some sense?
I could probably give a better answer if you gave a reference to a text (though it may not be all that clear in Epicurus' writing style if memory serves), but generically the need for food and the need for sex are pretty different.
The words "natural" and "necessary" can have several very different meanings. I take "natural" in the context of Epicurus to mean that which we have without any external imposition, i.e. things he things humans just have.
For "necessary" in this context, I take it to mean that without this thing an individual person (n.b., the inclusion of the term individual here) will not be able to live. In other words, that which is necessary here is that without which I will die.
I then take the two following sentences to be true:
If I were to not eat, I will die.
If I were to not have sex, I will not die.
Translating them into the terms of necessity:
Eating is necessary.
Sex is not necessary.
I want to eat.
I want to have sex.
become = I have desires for eating and sex. (and we can also adduce for most humans these are natural desires).
Thus, eating is "natural" and "necessary" but sex is "natural" but "unnecessary".
For the record, I don't think he's wrong.
"We must also reflect that of desires some are natural, others are groundless; and that of the natural some are necessary as well as natural, and some natural only. And of the necessary desires some are necessary if we are to be happy, some if the body is to be rid of uneasiness, some if we are even to live."
Epicurus in the Letter to Menoeceus (one of our only sources of this issue)
Desires are divided according to their relation to us as humans and the goal of living happily.
Groundless ones are those that are based on some kind of mistake. For instance, the belief that political power will bring with it true happiness... when in actuality it brings no end of problems.
Natural ones are the ones that are of true value to us as humans. Fulfilling natural desires does bring pleasure and happiness, without pains or difficulties.
Natural and necessary ones are, like the quote indicates, the ones that we MUST fulfill if we want to have happiness, comfort, or to live at all. These are the ones that all humans have to have anywhere and anytime. Eating is one of those that are necessary if we wish to live as biological beings.
Natural but not necessary ones are those that we can do without, and still have life, comfort, and happiness. They allow the personal variation and the variation over time. Sex falls into this category because the lack of sex doesn't prevent you from living, in comfort, and happily.
Epicurean Philosophy is a form of hedonism, that is, Epicurus says that pleasure is the criterion of goodness. However! The philosophy is NOT what we today mean by hedonism. In the philosophy, pleasure is the starting point on which is then built a system of morality so that humans can choose the natural desires over the groundless ones, and the necessary over the non-necessary. Epicurus starts by saying that pleasure is good, but continues by saying that some pleasures are deceptive (groundless).
In addition to this our vocabulary has been effected by both Roman prudery and Christianity. "Pleasure as the highest good" is from a text by Cicero... who once said that he'd rather be wrong with Plato than right with Epicurus. It's no wonder that he would distort the philosophy. Epicurean Philosophy and christianity are both 'missionary' worldviews: Both think that they would benefit all humans. They are also opposites in many other ways (e.g. Epicurean system is anti-supernaturalist, and promotes atomism.)
It's the opponents of Epicurus that linked the philosophy to hedonism, because it's an easy thing to take out of context and attack as a strawman.
As for sources of the philosophy, here are three. There are also Epicurean groups on Facebook if you wish to discuss with others who are interested.
- Epicurus and his philosophy, a book by Norman W. DeWitt.