Do dogs really defend their master, and do you think mine will?

Hey guys, okay, so lately i've had this weird paranoid feeling of going out anywhere alone. Don't get me wrong, i can be in places by myself, but with people around me if that makes sense, but i'm talking about, say, going for a nice walk to the park or some place where there is no witnesses to hear my screams if somebody were to attack me. I guess all those stories you hear about women being attacked, raped and/or murdered have made feel like its going to happen to me. My mom told me that whenever i want to go for a walk, and she can't cos shes too busy, to take my dog. That he'll know what to do if anything were to happen. Hes a 2 year old, male bordercollie, and hes my best friend. He knows me as his master, nobody else in my family. He'll be sleeping, and before i hear a noise or somebody talking, hes already heard it and is alert. Sometimes he does this little 'buff' sound (sooo cute). He won't let ANYBODY trespass on our property, he will go mad. When somebody goes past our house, he'll yip a couple times, and that's it. When a visitor comes to the door, and i tell him its alright, he'll settle completely down, but only if its a woman. If its a man, he will just bark himself out, whimper, and keep smelling them (mainly near their crotch, soooo embarrasing), while i have him by the collar and pull him back. Sometimes he shows his teeth. Only with men. In the past i've taken him for walks, and he'll be all friendly to people, as they past us, but then another person who looks like an ordinary person like the others that met my dogs approval walks by minding his own business, and he'll get so upset and look at me, and whimper, and then keep looking back as we walk on. Hes the only dog i've had so i have no idea why he behaves like this or whether its normal. But the fact of the matter is he has never bitten somebody, and so in the back of my mind, i think he'll never actually do it, if it ever comes time for him to protect me. Its like i'll feel more at ease, if he accidently did bite somebody to protect me, there would be that re-assurance, but of course by him biting somebody, it would mean he would be taken from me.