We've now got some on this thread saying that Jesus was not of Mary's seed. We have some saying that Jesus did not pass through the birth canal. We know that Jesus was not of Joseph's seed.
The question then becomes this: "In what way can we begin remotely to derive the idea that Jesus was fully human?"
This sounds like a gnostic preservation of Jesus "actually not" being human. Anyone who teaches this is unaware of gnostic influences that have entered their theology.
How is this not what John warns us of, "those who deny that Jesus came in the flesh?"
"The question then becomes this: "In what way can we begin remotely to derive the idea that Jesus was fully human?""
I think you point out a very real danger of getting too specific about the actual mechanics of the Incarnation. I have always maintained that the Latin Church's formulation of the Immaculate Conception, while likely a necessary consequence of the popular understanding of +Augustine's statements about Original Sin, posed the danger that one might conclude that Christ was not "True Man" as he was not born of a true woman, howeever wrong that conclusion might in fact be. The history of heresy unfortunately shows how often wrong conclusions lead to trouble. Sometimes divine mysteries are best left alone, though I suppose there is no harm in speculating about such things so long as both the speculator and his readers understand that is what is going on.
Christ is one Person, two natures, in perfect harmony, fully divine and fully humam, inseparably fused, but not confused.
My understanding is that God the Word took on human nature, using Mary's flesh in her womb, which He entered mystically without violating her.
The Church always taught that His mystical Birth likewise did not violate her.
He was not a pagan byproduct of some divine-human sexual union, a demigod like Achilles, a mixture of divine and human "seed."
Incarnation is an act of the Word of God, His will, and not a "natural" consequence.