Regarding your question, how Krishna can become a male: (your question,". . . I read from Srimad-Bhagavatam, chap. 11, text 7, 'Oh, Creator of the Universe, You are our mother, well wisher, Lord, father, the spiritual master and worshipable deity.' I was asked, 'How can Krishna be mother if He is the Supreme Male?' ") You have to forget always material concepts of existence for Krishna. Krishna is totally different from anything of material concept. In the material idea, we think that how a male can be a female, or how a female can become a male. This is material. But you know the picture that Krishna as Garbhodaksayi Visnu is lying on the sea, and a lotus stem is growing out of His abdomen navel, and Brahma is born there. Visnu's wife, Laksmi, was present there. She was just massaging the Lord's leg, and the Lord avoided her and produced a child. Here in the material world, without the assistance of the wife, nobody can produce a child. But there is the tangible example, that Visnu, without any sex life with wife He produced a child. In the material sense, we cannot conceive how a child can take birth without being born through the abdomen of one woman. Therefore, Krishna being both father and mother, He can be addressed also as mother. That is His all-powerful strength. He is full in Himself. He does not require anyone's help, therefore, He is both father and mother. I think this will clear your idea.
It is our material thinking that is hindering us. You are thinking male, of the material world, and a male in the material world cannot produce a child. And if you think also that Krishna is mother, again you are thinking with material conception. So we have to avoid using our material thinking and material concepts when we think about Krishna. Therefore, we have to approach a bona fide spiritual master in order to understand.
Another example is explained in the Bhagavad-gita, that Krishna begets the living entities, within the womb of material nature. The material nature is also expansion of Krishna's potency. The potency is not different from the potent. Just like the heat is not different from the fire. In other words, heat and fire is the same thing; so if we talk in that way, the material nature is the mother and Krishna is the father. But material nature being non-separable from Krishna, Krishna becomes both father and mother at the same time. The heat is fire and the fire is heat. Although you can bear the intensity of heat, but if you put into the fire, you will be burned. The heat and fire are simultaneously one and different. That is the philosophy of inconceivably one and different, expounded by Lord Caitanya. Try to understand in that way.