--I use the term semi-gnostic as the RCC seems to think that something as blessed as sex is wrong
-You would be well advised to better acquaint yourself with our theology of Marriage. Your statement, quoted above, 'seems' delusional and disconnected from reality.
("delusional and disconnected"? Heh!)
Then please explain why, if Mary's parents were married, she must be immaculately conceived herself. Her conception was pure if done the natural way.
Does the RCC suggest that Mary was conceived of the Holy Spirit similarly to Jesus?
What do you think "immaculately conceived" means?
"But the descendants of Mary are also sinners, going all the way back to Adam. She is not sprung from dust, she is human, thus carries the sin of Adam carried by all descendants of Adam except the one Perfect Savior.
Then please explain why, if Mary's parents were married, she must be immaculately conceived herself. Her conception was pure if done the natural way."
I will answer.
First, Mary had no descendants other than Jesus.
She was ever virgin, gave birth as a virgin, and remained a virgin until her assumption into Heaven. Nobody in the world is a descendant of Mary.
She WOULD carry the sin of Adam, but for the Immaculate Conception. If you wish to read the ancient texts that speak of the immaculate conception, of Mary's marriage to Joseph, and which give details about the Virgin Birth, the protoevangelium of James is available online. This is an orthodox work in the Catholic and Orthodox tradition, and was considered part of the canon of Scripture by many in the early days, though it did not end up in the final canon of the Bible.
Through God's intervention, Mary's conception in the womb of her mother was "immaculate", meaning that she was NOT infected by the Original Sin. God's power prevented the curse of Adam from passing to her. She was the first sinless person since Adam and Eve, and the first sinless person ever BORN (Adam and Eve were MADE). Jesus was the second.
Why was it so crucial for Mary to be without any sin, including (especially) Original Sin? The short answer is because wanted it so.
The longer answer would be that God abhors sin. For Mary to be tainted with original sin would mean that God would have to unite himself to sin, in her, and grow within sin, and be in full contact with sin during those long years. It would mean that God would take sin for his bride and consort and mother. And so He protected Mary from sin so that he could grow in her womb without being surrounded by sin, in contact with sin, DEPENDENT on sin for his very existence during the crucial period of being formed into a man by way of flesh instead of spirit, for JESUS was the only BEGOTTEN Son of God, God's first BORN son. God's first SON, first human son, was Adam (and that didn't work out so well...), but Adam was MADE. When God made Adam, he did not come into contact with sin, for the world was good and Adam and Eve both were made without sin. But by the time of Jesus, the world was marred by sin, and all humans carried the sin of Adam in them. God prepared a womb for himself to grow in by cleansing it of all sin, including original sin. God prepared for himself a bride, whose body He would overshadow by His power, by keeping her free from sin - indeed, the Protoevangelium tells us she was a Nazirite of the Temple! Obviously bearing a son by God is a very strong reason for Mary's perpetual virginity to be assumed, even required. Shall the Mother of God Jesus cuckold God the Father? Shall she, having been impregnated by the Holy Spirit, been the consort of God, abase herself to lust and adultery against God by joining with another man?
No.
And she didn't.
Mary was immaculately conceived because God did not want to grow in a womb contaminated with sin.