So, here is the thing...Masturbation is an evolved trait. We masturbate, as our ape cousins do to refresh our sperm so that the strongest can be allowed to attempt to impregnate. Do you know what happens if we stop having sex and masturbating alltogether? Wet dreams. It just so happens that masturbation is a far better method of training the new troops, so to speak, because It can happen more frequently. The point is, is that one way or another, your going to orgasm. Evolution has made sure of that.
I don't know why someone would get emotional over masturbation, except if they don't really understand what is or is for. me and my SO have a very strong sex life, And I suspect it's in part due to the fact that we know our own bodies, as well as each others. I've been with girls that 'don't masturbate', or at least not very often, and they seem to have a lack of knowledge of how to help me please them. If i'm trying to get her off, and she cannot help direct me due to her lack of knowledge of her own body, then how can I read her mind?
Also, it's okay to have fantasies. It's normal that there are things you'll want to do in bed, and she won't be into, and the other way around. It's okay to explore these things in your own mind, as it's really the place you have the most freedom. Telling somebody not to masturbate is like saying "there are certain thoughts I don't want you to have.". Only your SO is saying "there are certain thoughts I don't want you to have, except when I'm around...". Makes no sense, right?
As well, there are health benefits to masturbation. It can help you last longer in bed, it helps prevent cancer (ejaculating more than 5 times a week), it can help keep you harder as you age.
Talk to her, and help educate her as to what masturbation is for, and why we do it, and why we're lucky to have it.
TL;DR: Masturbation is a healthy function of any adult, and you should let her know this.