Anything short of perfection in His human nature — including His mother and His conception — would be incompatible with the notion that Jesus Christ was Divine.
So...God could prevent Mary from being tainted by original sin, but he couldn’t do that for Jesus?
Rubish. Christ was the bridge between sinful man and perfect God. He was both 100% man and 100% God. His 'perfection' came from His divine nature and was without original sin because His father was THE Father, not Joseph.
By extension, your logic implies the need for sinless ancestors, which is heresy.
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!