Someone said earlier that Lucas himself has touted the "virgin birth" idea in interviews. It's important to see that the virgin birth notion remains intact if you throw Sith into the mix. And just because the Sith manipulate the birth doesn't mean that Anakin was 100% dark side of the force.
Yoda himself acknowledges that the Jedi misinterpreted the original prophesy. The Sith likely knew how the Jedi were reading the prophesy and saw an opportunity within.
As for Darth Plagueis' age ... hate to tell 'ya this, but if the Sith history ain't in a movie, it didn't happen. Look how much of the Phantom Menace novelization is irrelevant in light of the films that followed Phanton Menace (re: Padme didn't lead an army against Anakin). Either way, if he trained Palps, then he could have been the sith who engineered Anakin's birth.
I hate geeking over details like this when there are so many other gaping plotholes in Revenge of the Sith. Plus, it's what Lucas wanted -- people quibbling over silly details that are neither creatively deep nor meaningful. In that, there's just no way Lucas would have tossed out this idea if he didn't mean it. The Sith manipulating the birth of Anakin fits their MO perfectly ... and I doubt Palps was lying in this one instance.