Why is every human being born a sinner according to this teaching?
According to this teaching, every human being is born a sinner because the curse is transmitted through the bloodline of the father. The human father passes on the inherited curse, so each person is a born sinner by inheritance, while the mother is equally a sinner but not the channel of the curse.
Les explains that the line of the curse comes through the blood, through the father. Therefore every human being is a born sinner by virtue of inheriting from the father, not necessarily the mother, although she is just as much a sinner as anybody else. He connects this to why God insulated the seed of the woman: Christ had to be born of a woman, yet be sinless. Since the woman's reproductive cells did not carry the curse, God impregnated Mary, and Jesus was born without the effects of the curse that came from a human father. Thus He was sinless and divine, while still human, born of the woman.