Sexuality wasn't a central concern of Confucianism, not as it is in Abrahamic religions. Propriety and temperance (as in, avoiding any physical excesses) are the main emphasis.

So, there's an old book called The Ethics of Confucius by MM Dawson that pulls teachings from a few different Confucian (or credited-to-Confucius) texts and organizes them by topic for Western readers. The section down the page here on "Sexual Propriety" ties a lot of different passages together, most of which a little... non-specific.

"The scholar keeps himself free from all stain," from the Book of Rites.

And: "To find enjoyment in extravagant pleasures, to find enjoyment in idleness and sauntering, to find enjoyment in the pleasures of feasting—these are injurious," from The Analects.

And even, from one of the commentaries on the I Ching: "Heaven and earth are separate and apart, but the work which they do is the same. Male and female are separate and apart, but with a common will they seek the same objects."

Men and women each have their own spheres of control, in other words, and shouldn't be mixing too much.

Dawson also quotes Mencius: "When a son is born, what is desired for him is that he may have a wife; when a daughter is born, that she may have a husband. All men as parents have this feeling. If, without awaiting the instructions of their parents and the arrangements of the intermediary, they bore holes to steal a sight of each other, or climb over a wall to be with each other, their parents and all others will despise them."

One of Mencius' big ideas was balancing internal (mmmaybe "spiritual") and external (let's call this "biological" for now) needs to cultivate the xin or "heart-mind". Elsewhere in the book that bears his name (Mencius(pdf), book 6A4), he has a dialogue with Gaozi (definitely another sage, probably another Confucian) that dips really briefly into biological imperatives:

Gaozi said, “Appetites for food and sex are part of our nature.

Humanity (ren) is internal rather than external; right (yi) is external rather than internal.”

Mencius said, “Why do you say humanity is internal and right external?”

Gaozi said, “If a man is my elder and I treat him as an elder, there is nothing of the elder about me. It is as if he were white and I treated him as white, I merely follow the external fact of his being white. This is why I treat it as external.”

And it goes on from there.

The bit about sex being a necessary appetite is really not remarked on directly in the exchange... they wind up talking about roasted meat, in fact. The appetites seem more or less morally the same.

Prior to that dialogue, (in Book 5A), Mencius refers to sex (or "taking a wife") as a "fundamental human relationship," but places it in a kind of hierarchy of how we ought to love:

“When we are young, we yearn for our parents. When we are old enough to have sexual desires, we yearn for youthful beauty. When we are old enough to have a family, we yearn for wife and children. When we are ready to take office, we yearn for a lord, and without a lord’s approval dissatisfaction burns within us. But the greatest filiality yearns for parents to the end of life. In Shun, I see a man who yearned for his parents even at fifty.”

So you can maybe see that earlier quote about the young lovers as an expression of "good" versus "bad" ways of dealing with biological drives versus personal cultivation and social mores.

Well after Mencius, the neo-Confucians were considerably more uptight, but I'll let someone with better chops get into what they were all about.