I am a monotheist.
I believe God is neither male nor female per se,
but has qualities which in Creation are manifest as separated genders - male and female, sometimes manifest in different beings (a man and a woman, a staminate and a pistillate spinach, asparagus, or rambutan), sometimes manifest in a single being (oyster, strawberry). So in a way, "my" God is more like a strawberry than a single human being, or more like a man and a woman than like two men or two women.
Understanding "gender" in the theocratic sense is not difficult.
As you said, it has nothing to do with physical gender, since there is no sexual gender in heaven. Sexual beings are those that reproduce sexually. Angels are A-sexual beings who have no reproductive powers because God created them individually. (The angels who materialized in Noah's day are a whole other story)
Jesus confirmed their A-sexual status when he was questioned by the Saducees about the resurrection....
"In reply Jesus said to them: “You are mistaken, because you know neither the Scriptures nor the power of God; for in the resurrection neither do men marry nor are women given in marriage, but are as angels in heaven."
Angels do not marry or produce children in heaven.
Humans are reproductions of themselves. We are purely the product of our parents. Their gene pool, becomes our gene pool. Before we were conceived by them, we did not exist.
Like all other living things who share our planet, we are mortal beings who rely on external things to keep living (air, water, food shelter etc) But humans alone were made in God's image. We alone were promised everlasting life right here on earth.....but our first parents messed that up for us. (Gen 3:22-24)
In God's arrangement on earth, there is one appointed to carry the responsibility for the care and well-being of his own family unit. Each family is under the headship of the "man of the house". (1 Cor 11:3)
In God's arrangement then, women have a support role, as God told Eve. She was to be a "helper and compliment" of her husband....not his slave and not his servant. But someone who complimented his role and assisted him in the smooth running of the household.
His headship was not to be a dictatorship, but imitating the same caring and loving headship that Jesus manifested towards his spiritual family of believers. He loved them enough to offer his life for them. (John 3:16) Their welfare both physically and spiritually was his assignment.
To make God into a female in the eyes of humans would be to run contrary to his own headship arrangement. The "maleness" wasn't just in the gender, but in the position that God had assigned him as head and decision maker. Making God female would have undermined the whole arrangement.
We are all made in God's image...meaning that we can all reflect his qualities such as love, justice, wisdom and power within the capacity that he has assigned to each of his human children....both male and female.