Obviously the out of universe reason is that Star Wars is PG. But I believe that the in universe reason is this:
After the rule of two was created, Darth Bane decided that the Sith must remain covert. However, with the Sith no longer having the resources that they once did, their alter egos often are quite rich, influencial and well known (Darth Sidious, being Supreme Chancellor is the perfect example). Therefore I believe that the Sith do not use sex to fuel their power, (primarily) because it would look bad for their alter ego, should the public ever find that they were sleeping around. Some of the lesser known, rule of two Sith lords, may very well have used sex to fuel their power and we just never hear about it. Namely Darth Ramage ;)
Also, many of the Sith that we see just don't seem interested in sex. The Sith, like the Jedi, dedicate themselves to their Order and make sacrifices for it. The Sith are tortured for most of their training. In other words, the Sith seek power for the continuing of the Grand Plan not just for personal gain.
Sex is not only passionate but also pleasurable and for the Sith to engage in it would seem like to much of an indulgence to be justifiable. Hego Damask a.k.a Darth Plagueis, doesn't even like eating gourmet food, this is partly because of his race's weaker olfactory but might also be because he values discipline and associates indulgence with weakness, this is evident when he tells Sidious not to give Maul so much praise and when he learns to go without sleep so as to spend more time working.
Darth Tenebrous had a young (and apparently attractive) Muun disciple but there is no evidence that he ever engaged with her and indeed she is seemingly discarded after she gives birth to Plagueis. This is likely to be because he thought it unfitting that he should engage with someone of lower status. It is easy to imagine that this is why none of the Sith ever seem to have sex, because no one is good enough.
I do not known too much about the Sith before the rule of two, perhaps they did engage in sex, I have not seen any evidence otherwise, especially in the lower ranks. If they didn't, however I believe that their sense of discipline and superiority were the contributing factors.
I hope that at least partially answer your question.