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:
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 man can be used to refer to mankind, instead of just male.
Here are some thoughts I have had over the years:
Of course that is pure conjecture on my part and there are no Scriptures for me to cite, but there are some things that I believe God just didn't think we needed to know before we go to Glory.