This is an argument for the Immaculate Conception (sinless nature) of Mary that works a bit differently than most. It goes like this.
Jesus has no earthly father, which means that his entire humanity has to come to him from Mary. But problem! If Mary is contaminated with the stain of original sin and Jesus is not, then something had to happen in the transmission of his humanity to alter it. This would seem to put in doubt the connection between Christ and the rest of humanity, as it would mean he not only had a single parent, but even what that one parent contributed to his humanity was not fully inherited by him, so it is almost as if he had only half a parent. This is not a knock-down argument, of course, but it would be extremely unfitting for Christ to have such a tenuous link to the rest of humanity.
Granted, Mary herself had to receive a “modified” humanity from what her parents gave her, but this does not risk the sense that Christ is related to the race by a only slender thread.