We're dancing here.
"Because he is God" does not mean that anything goes. God is light and light has nothing in common with darkness. There can be no evil where God dwells for God is all holy. To the extent that God is present, evil is not and vice versa. Think of it in the same way as one of the laws of physics.
Just "because he is God" does not mean that God can suspend His own Divine, holy nature to partake and receive sustenance from that which has been tainted by sin. To suggest otherwise is to infer that God's nature is not "all holy", which I think we're both agreed, is untrue. To do so would be to go against the fundamental nature of God.
People use the term "because he's God" unthinkingly to imply that God can do whatever. It's false. God does not sin, for example. Not only that, sin cannot exist in the presence of God. The two are incompatible.
That is why the womb which bore Jesus must have been sinless.
“Because he is God” is not a statement declaring God has/can/or will ever suspend his divine and holy nature. As you rightly stated, this would go against the fundamental nature of God.
The rational mind concludes that if God’s Son was carried in the womb of a sinful woman, receiving genetic material from her, he would receive her sinfulness. But if you require that Mary had to be sinless to give birth to a sinless Son, then you would have to require the same of her mother, and her mother, etc. Not even Mary saw herself that way. Remember how she rejoices in “God my Savior” in Luke 1:47? If she was immaculately conceived without any sin herself, then she didn’t need a Savior!
This discussion is very similar to the following question:
If Jesus is true God, how could he die, for God cannot die?” These are questions we can’t answer but simply have to accept in faith.
If she, Mary, was sinless as you are describing, wouldn’t she need a sinless womb herself. Lets not even delve into the sin state of her father for the moment.
If a sinless womb is needed, then there would need to be a chain of sinless wombs dating back to creation.
If she did not need a sinless womb to be sinless herself, why would you argue that Jesus, the very son of God, needed a sinless womb?