1) Guns are a form of protection.
Shields, armour and helmets protect you. Guns shoot toxic pieces of lead into other people.
2) Guns are a form of self-defence.
They can be, sure. However, statistically speaking, more guns are used in crimes than are used in self-defence.
A study of gun use in the 1990s, by David Hemenway at the Harvard Injury Control Research Center, found that criminal use of guns is far more common than self-defense use of guns.
Obviously, potential criminals are going to purchase guns if they are legal. Therefore, you make a useless trade: increased personal protection for increased need for it. The only logical outcome of that trade is more people will die.
3) I don't trust the government.
If the government wants to get you, then your Dirty Harry handgun is not going to stop them. End of story.
4) A 100 lb woman shouldn't have to struggle with a 300 lb rapist.
But you can't arm the victim without simultaneously arming the rapist. So, instead of a rape occurring, which is consequently then investigated by the people whose job it is to investigate rapes, you have a bloodbath. Maybe the rapist gets killed, maybe the victim gets killed, maybe they both get killed, or maybe innocent bystanders get killed.
There are alternative ways to deal with rapists which are final, much safer, and do not end in loss of life.
5) Guns don't kill people. People kill people.
Then it is a stupid idea to give them guns then, isn't it? Unless you are trying to make fucking sure.