To satisfy your curiosity, I can explain very plainly. But since Christians believe that Jesus was born of a virgin by the Holy Spirit, and God's primary object of this dispensation is Christians (it is His will that we lead Christians first because they are the ones who have been keeping the promise of the Second Coming), it is our mission to tell them first, whether they refuse or accept. If I explain everything plainly about the birth of Jesus, it may put a stumbling block in the way of many Christians accepting God's new dispensation. Therefore, I don't want to explain everything too plainly here.

Master Speaks (1967) Part 7: Bible Interpretation (transcriptions of Q&A sessions in March-April 1965)