I think you misunderstood my meaning. See my explanation to CG.
If you can believe that God placed a fertilized embryo in Mary's womb without using any of her genetic material, why can't you believe that God preserved Mary from original sin?
Because there is a prior scriptural example of a human being coming from something other than human procreation, namely Adam. There is however, no accounting for a suspension of original sin in any human being (resulting from procreation between two human beings) anywhere else in Scripture.
If God can just selectively suspend original sin in humans, why send His Son to die?
Catholics aren't required to limit themselves to only Scripture when that itself is not Scripture.<>
We do not merit salvation. We are creatures of the flesh who deserve eternal death and are unworthy of God our Father. But in His infinite mercy and compassion, He sent down His only Son to not only die for our sins and our salvation, but to teach us to take up our cross and follow Him. If God had just removed the stain of original sin from our souls, we could still fall like Adam and Eve through the temptation of Satan, because humans are creatures with Free Will (just like the angels).
Why didn't God just come down Himself and throw some lightning bolts at the heathens where everyone could see? Why didn't He just kill off all the sinful people and start with the good guys? Why didn't he destroy the earth and start all over on, say, Venus? The fact is, we don't know. What we DO know (and agree upon) is that Jesus is the Son of God. He walked the earth and told people how they should live their lives and worship the Lord. He told his Apostles to preach the Good News of the Word of God. We know that he was crucified, died and was buried, and on the third day rose from the dead. We know that he performed great miracles before and after his resurrection. We know that he the second person of the Trinity. We know that he died for our sins, and that we do not merit salvation, that nothing but the grace of God gives us eternal life (as Catholics would say, we believe in cooperating with the freely given, unmerited grace of God because, being creatures of free will, we can accept or reject God's grace.)
So, I don't have all the answers, but fortunately I don't need them all. I have faith that Jesus rose from the dead. That's a pretty overwhelming belief, really, when you think about it, but I know it happened. Just like I have faith that Jesus Christ established the Catholic Church to spread the Word, and the gates of Hell would not prevail. The early church fathers believed in the Immaculate Conception, and the Church has always taught it even though it wasn't "formally" defined until later. That's good enough for me.
I know that there are some things we will never agree on, but I know that there are many things that we do share. God bless you.