The point of Anakin Skywalker's birth is that, ultimately, the Force conceived him. Like the virgin Mary had Jesus without having ever known a man, Shmi Skywalker had Anakin without a father. It is debated whether or not Darth Sidious or Darth Sidious' master Plagueus was the one that caused the Force to do so, but ultimately, there was no physical human man that impregnated Shmi Skywalker. The Force caused Anakin to spontaneously be created in her womb.
There as been some speculation that Anakin was created by Darth Plagieus. However, this does not make sense, as Anakin was born at least 3 years after Plagieus was killed.