The Force in the original trilogy is essentially California-style, avocado-flavored magic. Midi-chlorians were clearly Lucas' attempt to bring the Force into the science-fiction fold. It was also a way for him to explain a “virgin birth.”
If midi-chlorians were only a metric by which Force strength is measured, it wouldn't have been an issue. Unfortunately, the explanation of midi-chlorians make the Force lose some of its mystical quality, and the “virgin birth” is not only pulled by the hair in TPM, but doesn't pay off in ROTS in any meaningful way.
The Force could have stayed supernatural and nobody would have minded, and Shmi was a slave, that's all the backstory she needed for having a child and not knowing who the father was. But I think Lucas considers sex to be icky or something, so he managed to come up with a way that she could have a child without spreading her cooties, or whatever it is he thinks women do.