My Dog is scared of all men. Help?

I have a 4 yr old black lab that is scared of all men and I don't understand why. We got her from our neighbor after their dog had puppies and shes lived with me, my mother and my brother ever since. She runs from my brother and does everything to avoid him. Now that I'm living with my bf, its becoming more of a problem because we have many male friends that she freaks out about and my bf can't touch her without her nipping at him. Hes starting to dislike her now because of it but he his willing to help if there is a solution to this problem. What can I do to get her to accept men? She loves women but just not men.