The Sith had grown so powerful they apparently managed to manipulate the force as a whole to create the goddamned Jedi Jesus for them to corrupt. They weren't working within the natural order of things, they were throwing rocks in the pond and diverting the river for their own ends.
Well, at least that's what Sidious and Plagueis believe in the now decanonized novel Darth Plagueis.
At the risk of starting up the crazy argument again, ROTS and the PT generally (and the OT for that matter) really say nothing one way or the other. Nor do they imply anything, where by imply I mean necessitate.
The PT and the OT are perfectly consistent with either idea, on the one hand of Anakin having been created as a result of Plagueis's life-creating experiments, and on the other of Anakin's creation not being such a result and instead having (what are as far as we presently know) purely mysterious causes.
Certainly, the Darth Plagueis novel was one of the brighter spots of the old EU, but frankly I didn't find it fully satisfying either. In any case, that was only one perspective on Anakin's birth. Based on what's been revealed in The Clone Wars, such as in the Mortis arc, no doubt there are other differing perspectives. Perhaps the new canon going forward will weigh in on the matter.
Yeah, I always found it a dubious claim regarding Plagueis, especially afterwards when Sidiuous is all "Only my master really knew it, but we can figure it out together." Then I got suspicious.
I have no doubt that the Sith can use the Dark Side to manipulate the Force and cloud the light side users abilities. But, I have a tough time believing that the Sith can create life without corrupting it in some way.
That's just my take though.