Because the Sith only look inwards, for themselves, and for the source of their strength:
PALPATINE: From a Jedi's point of view. The Sith and the Jedi are
similar in almost every way, including their quest for greater power.
The difference between the two is the Sith are not afraid of the dark
side of the Force. That is why they are more powerful.
ANAKIN: The Sith rely on their passion for their strength. They think
inward, only about themselves.
PALPATINE: And the Jedi don't?
ANAKIN: The Jedi are selfless … they only care about others.
--- Star Wars: Revenge of the Sith
... and because their ultimate goal, as per the Sith Code, is to free themselves utterly, from anything that can limit or control them, to break every chain. However strong sex may be as a motivator, it still implies desiring, or even needing, something outside yourself, in order to satisfy it, or achieve it, which is anathema to the Sith who only think of themselves and want to be free of everything else.
Note the example of Darth Malgus and Eleena Daru. Malgus had to keep his relationship with her a secret lest the other Sith find out and use it, or her, against him. At one point, he found out that she was in trouble and he went to rescue her, but after having done so, he realized that he wasn't acting to his own power, he was acting to save the life of another.
From his Wookieepedia article:
However, Malgus realized his worst fears and his experience with Aryn
changed him for good. He realized that Daru was his weakness just like
the Jedi. He could no longer afford this weakness. Therefore, he
killed his only friend and freed himself from the shackles of
He realized what a weakness she was to him, so he killed her, then used his pain and anger relating to her death to fuel him and make him even stronger in the Dark Side.
Again from his Wookieepedia article:
Prior to duel [sic] with Adraas, Malgus became more powerful than ever
before by eliminating his only source of weakness; Daru. Her death
allowed Malgus to reach in such a mental state that he resided in the
calm eye of a storm of hate. Power churned around him, within him. He
did not feel as if he were drawing on the Force, using it. He felt as
if he were the Force, as if he had merged with it. Within this state,
Malgus became virtually unstoppable.
While the Sith do use passion to fuel them, the purpose of their passion is to give them strength, and from strength power, and from power victory, and from victory freedom as per the Sith Code:
Peace is a lie.
There is only Passion.
Through Passion I gain Strength.
Through Strength I gain Power.
Through Power I gain Victory.
Through Victory my chains are Broken.
The Force shall free me.
Passion isn't an end unto itself, it is only useful to the Sith insofar as it leads to strength. Any passion that they have which could theoretically weaken them, or risk their strength, and thus their power, they would see as something to avoid. Whereas hate and anger and a desire for more power come entirely from within themselves, lust/sex requires them to depend on something outside of themselves, and even if they don't love the person they are with, they are not free as long as they need them to fuel their strength.
A last example of this, related to sex, is that the Sith desired immortality, not offspring. Their lust for power to achieve freedom was so great that it overcame those same primordial instincts (that you mention above) to procreate. Their desire was to free themselves from the ultimate chain, Death itself, rather than continuing themselves in a reduced and indirect way through having children.