If you want a rational explaination why the Immaculate Conception didn't happen, how about Occam's razor? The way to salvation is through faith in Jesus. Why add an exception when it isn't needed?
The prophesy about Jesus' birth requires a virgin of the house of David. The gospels talk about a virgin of the house of David. Anything else isn't discussed or required.
When the Good News is discussed, it mentions a virgin birth. The creeds mention a virgin birth.
According to Occam's razor, no additional details are required and therefore didn't happen. QED.
Man, leave a thread about St Paul for a few days and it's turned into a thread about Mary.
I secretly think all protestant-catholic threads end up this way.