First put it in the form of a valid argument:
(1) There is only one God
(2) Mary was impregnated by God
(3) Mary's Son was God
Therefore
(4) Mary committed incest.
The problem with this argument is that it is not a *valid* argument (where 'valid' means that if the premises are true, then the conclusion must be true). But the argument I presented in #1926 is valid.
-A8
Your argument isn't valid because it assumes (falsely) that Mary Mother of God is the only valid conclusion. It is a false dilemma.