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.
Letter to Sacisuta, 17 June, 1968