Why are all angels masculine? Nowhere in the Bible are angels feminine.  If angels are messengers from God, why do these messengers always manifest themselves as masculine?  Is it because they assume the gender of their Divine dispatcher?
As far as I know, angels are different beings from us as they do not procreate. Matthew 22:29,30 says:


  Jesus replied, “You are in error because you do not know the Scriptures or the power of
      God. At the resurrection people will neither marry nor be given in marriage; they will be
      like the angels in heaven.


Because they do not procreate, gender really is irrelevant.

EDIT: I will add; I believe that in the Bible they take the shape of a man, but as in human shape. We know not if this is a female or male shape. Just like in the Garden of Eden:


  So God created man in his own image, in the image of God created he him; male and
      female created he them.


So man can be used to refer to mankind, instead of just male.