There are things you learn from reading the Bible, but to really understand the Bible you need to listen to the Lord and spend time with Him. There is something written in the Bible about how all Scriptures were inspired by God through the Holy Spirit and a conclusion coming from that which tells us that because of the Holy Spirit inspiring the Scriptures we must get the interpretation from Him - then not by leaning on our understanding - right?
It is written that His sheep hear His voice, and that He (Jesus Christ - the One who sends the Holy Spirit with words for us) is the teacher. Anyone having a teacher understands that some things are taught first then more and more are later taught. So it is that first a person must come to know our Lord Jesus Christ. Yet Jesus Christ is the King of kings, so He has a Kingdom - the Kingdom of God. So at some point are we not also taught about the Kingdom - and that from the King? Of course it is also written that He sends His ministering angels to those who will inherit salvation - so part of being taught by Him is His sending His Holy Angels to us! At some point, if we practice listening and using our spiritual sense we should have gotten more information about the Kingdom of God!! Perhaps even an explaination and a better understanding of the Kingdom which would explain the fact that Zechariah saw female angels and also why in heaven they are neither male nor female! The explaination might even work in with other verses in the Bible.
Of course, we also all know in part, but let me give you a little part I have come to understand. Angels are neither male nor female but they are spirits! As such they do not have a "physical" appearance!! So when we see angels (spirits) with our spiritual eyes they appear to us how we interpret the impression left on our spiritual eyes (which are not physical) So if an angel appears to you as female or male it is not because they are female of male but because you are picking them up with you spiritual eyes as being female or male! Have you not read that Satan might appear as an angel of light? That is to say Satan can show up to you seeming to be very holy and righteous, because He deceives you. If you test the spirits then, and find out that a spirit is in fact an evil spirit (like Satan) the appearance of that spirit instantly changes to you!! It might be something like in the Harrison Ford movie, Raiders of the Lost Ark, where at the end they open up the ark and spirits come out looking beautiful and then change to looking evil.
The very fact that this is seen in a movie shows that people have a rough understanding already in them, about what I am saying! That is because while we don't pay very close attention to the spiritual things, but rather spend all our time thinking about the flesh and the physical things, there is still some understanding in us, because we are spiritual being living around spiritual beings.
So why does a spirit appear female as opposed to male? We might also ask why the name Jesus have a female root. At least I read that it does. The answer has to do with being in authority or under authority. Thought the Christ (Jesus) is the King of kings as appointed by the Father (See Ps 2) He still came under authority by taking on flesh, coming to earth, and submitting to the cross. Indeed, that act means salvation to us, and Jesus means God's salvation. Now God made man then woman, and the woman was, (generally speaking) place under the authority of the man. (note: I am not trying to be sexist and I don't have a problem when God puts a physical woman in positions of authority - say like Deborah in the Bible). Still, because men are generally thought of as being in authority and woman as being under authority, the appearance of an angelic being seeming to be female tells you something about how they are presenting themselves to you.
Let me give you some examples I got from the Lord. He once sent me 40 angels that ministered to me, and I was to figure out their names. It turned out that they were name for what they generally did. The angel leading the group was Praise. Judah means praise and was often place at the head of the Israelite army. It is also why we often see Praise coming first in a church service. Praise appeared a male angel, and a very handsome male, and kind of like what I might call "a pretty boy". He had bright blond hair with thin appealing features. To His right was an angle named Joy, who was also good looking, but taller and stronger - much stronger. It went along with the phrase "The Joy of the Lord is our strength". Now both were spirit so neither had a physical appearance. Thus, there appearance was how I picked the up in my spirit. There were neither male nor female, but they did have a clear appearance to me. As I went through the 40 angels, most appeared to be male, being in positions of authority. Some appear more masculine. Obedience look very masculine, like a guard on an NFL team. Obedience was actually very nice but he look intimidating. Now some of the angels appeared to be female to me. Mercy and Grace were to who always appeared as females to me. They had kind of a motherly look, because in our lives we normally find mercy and grace coming from our moms, right?
The point is that angels are spiritual beings! They don't have physical bodies! And since we see them with our spiritual eyes we get impressions not from light waves bouncing off them, but from what ever we pick up in our spirit. Peace was an interesting angel because sometimes the angel Peace appeared to me as a female and sometime as a male. The appearance changed based upon the message he came with from the Lord. So if Peace seemed to be a male, then I understood that the Lord had sent him with a message that had authority behind it. One of the primary messages that Peace comes with from the Lord is almost an ultimatum, saying that you need to make peace with God or else. However Peace also comes with a message that you have the peace of the Lord to use it as needed, and in that case peace might appears as an angel under authority or giving you authority from the Lord - thus as a female.
It is just an appearance, but having spent 20 years walking and talking with the Lord I have gotten a better understanding of the King of kings and His kingdom, which is not of this world, than I used to have. Still, there is way too much for me to learn. Indeed, I have forgotten so much of what was minister to me by the Lord and from those angels He sends to me. Trying to hold onto all the information I have been given is like trying to hold onto the wind. Of course it is also a bit like drinking water. I need to drink water everyday, and it just seems to go through me, but somehow it adds to my living everyday. So drink the cup of the Lord that He gives you, which is Himself and His Kingdom.