Well I would agree that male and female genders are part of our being, because this is how we were created. We were created to be in relationship, the fruit of that relationship being new life. We are pro-creating, cooperating in God’s creation. While our gender makes us who we are (part of our being) it is, nevertheless, necessarily linked to sexuality for the purpose of producing new life. The concept of “Heavenly Mother”, by its very nature, implies that Heavenly Father is dependent upon “Heavenly Mother” in order to produce children, whether in a sexual sense or in a spiritual sense. It also implies yet another god, or goddess. I believe that God is not dependent upon anyone or anything in order to create.
Yes, as I said above, God created us in His image, that is “relationship”. You are quite correct that He could have made us any way He wished. He chose to make us in His image, as persons who are in life-giving relationship with each other, just like the Trinity. Gender, however, is inextricably linked to sexuality, regardless of its other wonderful attributes. “Heavenly Mother”, then, implies sexuality. That is the problem with the concept. Because there is only one God, He cannot be limited by gender.