God as a mother?

In worship a few weeks ago, we sang the hymn Praise to the Lord, the Almighty (UMH 139).  Verses 1, 3, and 5 were written by Joachim Neander in 1680.  Verse 2 was written by S. Paul Schilling in 1986, and verse 4 was written by Rupert E. Davies in 1983.  I didn't know that until this moment, and I find that very interesting.  

The fourth verse of the song includes the phrase, "Then to thy need God as a mother doth speed, spreading the wings of grace o'er thee."  

Never mind that two modern authors used words like doth and thee to fit there words into the ancient sounds of verses 1, 3 and 5.  Davies verse, which is quoted above, has an outlook that I have always considered appropriate but modern.  "God as a mother," are not words without controversy.

As I was singing the song, though, I thought that those words might not be so modern after all.  Why do we have preconceptions of what is maternal and what is paternal?  Other than the biological, why do we only consider women to be nurturing?  My husband definitely nurtures out sons.  When we cast our stereotypes on God, doesn't that box God in?  

I completely agree that God nurtures, and that God covers us with wings of grace, but why would we only think  of those as feminine traits?