Ultimately, Sith aren't dedicated enough to raise a family. Although they relie on emotions to strengthen them, their main focus is the persuit of power. Part of that is killing everybody in your way, even if it is your spouse or children (think Greek/Roman mythology). Plus, sith have a large lust for blood. Spoiler: A certain companion mentions when they were dark side, they killed two siths by the age of eight. Dead people don't reproduce, and there are a lot more dead Sith and alive Jedi than vise versa, because Sith kill each other.
Many great Sith Lord kill for a purpose.
Since they don't have the great bloodline and many of them born in poor families, but was able to bring so much trouble to the galaxy, does that mean Sith are more powerful than Jedi as individual ?