HP: We both agree that Christ was born of a virgin, but a seed was implanted of the seed of Abraham.
It sounds like you have a misunderstanding of Scripture or a denial of Scripture, I am not sure which one. Go back right to Gen.3:15, the very first Messianic promise. Christ would come of the "seed of the woman." This is significant because it is not the woman that has the seed, it is the man. The seed was not provided by Joseph, Abraham, or any other man, but rather by the Holy Spirit of God.
The Scriptures tell us plainly that Mary was "conceived of the Holy Spirit." Go to church at this time of year and maybe you will hear a sermon on that subject. 
Christ was not conceived by a man, but by the Holy Spirit.
Scripture also tells us clearly that Christ, from a physical perspective, was indeed a descendant of his father Joseph.
Then quote the Scripture.
Scripture simply does not tell us why He was birthed as he was, or that he had to be because of original sin.
It is the only way that he could escape the curse of Adam.
It is also the only way that he could escape the curse of Coniah.
I fully understand why you have to say that. To be fair, you have to admit that is simply not stated or implied by Scripture, but is a philosophical position driven by the presupposition of original sin.
I have Scripture; you do not.
If you disagree, show us from the Word of God where it tells us 'why' He was born of a virgin, and 'why' a human seed could not have been implanted by the Holy Spirit.
And I will put enmity between thee and the woman, and between thy seed and
her seed;
it shall bruise thy head, and thou shalt bruise his heel. (Genesis 3:15)
--The seed of the woman is Christ. This prophecy is given in the midst of the curse put on Adam and the rest of creation. The very context denotes that the virgin birth, here referred to, was necessary to avoid the sin nature being passed on.
Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel. (Isaiah 7:14)
--An absolute necessity.
But when the fulness of the time was come, God sent forth his Son, made of a woman, made under the law, (Galatians 4:4)
--Made, not born. A reference to the virgin birth.
But while he thought on these things, behold, the angel of the Lord appeared unto him in a dream, saying, Joseph, thou son of David, fear not to take unto thee Mary thy wife:
for that which is conceived in her is of the Holy Ghost. (Matthew 1:20)
--These words to Joseph indicate that he had nothing to do with this. Christ was conceived by the Holy Spirit, as it clearly states here.
Now all this was done, that it might be fulfilled which was spoken of the Lord by the prophet, saying, Behold, a virgin shall be with child, and shall bring forth a son, and they shall call his name Emmanuel, which being interpreted is, God with us. (Matthew 1:22-23)
--The fulfillment of prophecy; but it was more than that.
And knew her not till she had brought forth her firstborn son: and he called his name JESUS. (Matthew 1:25)
--Jesus means Saviour, or "Jehovah the Saviour"
The Saviour could only be a Saviour if he himself was sinless. He could only be sinless if he had no sin nature. He could only be without a sin nature if he were born of a virgin, as the record states.
And, behold, thou shalt conceive in thy womb, and bring forth a son, and shalt call his name JESUS. He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David: (Luke 1:31-32)
--born of a virgin, not of a man. Why? To escape the stain of sin.
Then said Mary unto the angel, How shall this be, seeing I know not a man? And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God. (Luke 1:34-35)
--No man was involved because "that which shall be born of thee shall be called the Son of God."
Is it not evident that God can have no sin? If there were a man involved there would be the taint of sin; the sin nature.
And my spirit hath rejoiced in God my Saviour. (Luke 1:47)
--In this one short verse Mary admits her sinfulness and her need of a Saviour. Her life had been tainted by sin simply by her birth (born of a man), no matter how well she may have lived it. But not so with Christ. He not only lived a sinless life. He was born without a sin nature. Never did Christ have to refer to a sin nature, or even infer that he was sinful, like all that went before him.
Is this man Coniah a despised broken idol? is he a vessel wherein is no pleasure? wherefore are they cast out, he and his seed, and are cast into a land which they know not? O earth, earth, earth, hear the word of the LORD. Thus saith the LORD, Write ye this man childless, a man that shall not prosper in his days: for no man of his seed shall prosper, sitting upon the throne of David, and ruling any more in Judah. (Jeremiah 22:28-30)
Coniah was a descendant of David. But in his transgression he was so cursed that neither he nor any of his descendants would ever sit on the throne of David. Joseph happened to be one of the descendants of Coniah. For this reason also Joseph was unable to be the father of Jesus, and Christ was virgin born. Joseph was under a double curse: the curse of Adam and the curse of Coniah.