According to your own private notions of human logic.
I really don't understand exactly how God the Son was born to a Jewish virgin. Do you?
All I know is the holy Scripture tells me this is true, so I accept it.
To conceive of a trail of human logic behind that, and then do demand its acceptance as fact, puts your own logic above and beyond the revelation of the Holy Spirit in the bible.
And what about St. Anne? Wouldn't she also have to be sinless to have an immaculate conception of Mary? Oh wait...that's a MIRACLE!
The Church proceeds the written New Testament.