Hm... I suppose I'll try to explain this.
Personally, when I pleasure myself, it is a time of pleasuring MYSELF and no one else, which means that no one is going to watch me do it or hear about it later. Masturbation, to some women, is a time to completely self indulge, a time that they do not feel is necessary to share with men, or anyone else, for that matter.
Yes, women masturbate. No, women do not need to share it with men. To share it with a man is no longer a private self indulgence. It becomes something the man twists around in his head to become his perverted fantasy, or something the man feels he needs to learn more about.
I made the mistake of telling a man I masturbated once. He began asking question after question. How do I masturbate? Do I use any toys? What do I think about? I felt like my personal fantasy, my self indulgence had been tainted, made into something perverted for a man who could not understand what I was truly feeling, thus taking away the one thing that was completely mine to keep from the world.