And Jacob begot Joseph the husband of Mary, of whom was born Jesus who is called Christ. [Matthew 1:16]
Reading the first chapter of Matthew, the first of the 4 gospels, one will notice that the genealogy follows a pattern of A begot B, then B begot C. This allows us to trace the family line. However when it comes to Joseph and Jesus, it was phrased differently, the first indication that though Jesus was born in Joseph’s family, Joseph was not the biological father. A few verses later, Matthew explained that Mary conceived before their marriage and the baby was child of the Holy Spirit.
If Joseph begot Jesus, then Jesus would not be perfect enough to be the Lamb of God.