Your argument isn't valid because it assumes (falsely) that Mary Mother of God is the only valid conclusion. It is a false dilemma.
You have obviously never taken basic logic. My argument is valid because *if* the premises are true, the conclusion necessarily follows. In the science of logic, that is just what we mean by 'valid'. (Conclusions are not valid or invalid; *arguments* are valid or invalid.) If M is the mother of P, and P is God, then M is the mother of God. If those two premises are true, then that conclusion cannot be false. I am not offering any "dilemma", let alone a "false dilemma". I am presenting (in #1926) a valid deductive argument. The only way to refute it is to reject one of the two premises.
-A8