I have no idea why masturbation is ever thought of in a negative light. It is in no way dirty, sick, or perverted -- it is totally natural, something we are designed to do. How can there be anything wrong with something that feels so good and hurts nobody? I think everybody masturbates -- certainly all men, and probably most women. I do. My partner does. And we both enjoy talking about it, and doing it for each other (both remotely and in front of each other). I love the shared intimacy and I think there is nothing hotter than watching a guy stroke himself, or hearing about it from him after the fact. However, it is indeed a very intimate act. In my younger years I never admitted to anyone that I masturbated, and I never asked a man if he did. I think that is due to social conditioning plus the fact that it is a profoundly personal act. When you admit such very personal things to another person, you are inviting them inside you in a way that can make you feel vulnerable. That feels good with somebody you really trust, but maybe not so good if you are unsure of yourself.
Anyway -- I certainly did not feel badly when my man told me about his masturbation. He's a man -- I knew he did it. I was just glad he was willing to talk about it with me. However, I am a totally grown up woman and we were very close before we started talking about such things. My advice if you want to start talking to a girl about this is to go slowly. You should probably introduce the subject by asking her sometime if she's like to watch you stroke yourself (I think that is a secret fantasy for most women). Once you get past that and it is a regular part of your sex play, you can think about telling her what you do when you are alone. I suspect that most girls have no idea about the extent of the sex drive of most young males, and how often they need release. You can tell your girl, but go slowly and don't get outside the comfort zone for either of you.