Hi, I think you would agree that it's a little hard to stop after experiencing it. I agree you need to get and be completely open about it with her. Tell her you've been there...done that, you know how good it feels and get down to her level. There is nothing "weird" knowing your kid is masterbating. It is normal. Tell her what it was like for you (wether it was or not ), that it's normal to get horny and normal to masterbate. Masterbation is a great way to learn about your body, what feels good, learn to enjoy it without shame, etc. Ask her if she does and how often and offer some advice on how to do it, and how to get more pleasure out of it....BECAUSE, the alternative is the sex you don't want her to have. Explain to her the down side of sex at her age...the emotional attachments that follow at an age too young to really deal with....all the broken hearts, possible disease and pregnancy...and explain to her that teen age boys are mostly concerned about their sexual needs which means she will get alot of heart break from alot of different boys cause the relationships don't last. Her need for relief will just lead to alot of different guys and heartbreak.
Just be honest and ok with yourself telling her the truth and all the reasons it's better to become good at masterbation and do anything you can to help her out with it. Tell her (and you) horny is normal but masterbation will give her alot of pleasure if done right and anything that helps her learn that within reason is a good thing. Most girls don't know much about their bodies and orgasims. If somebody can teach her the tricks and tips and encourage her to find things that really help her...most boys would be a sexual dissapointment to her after she masters it......which will help her stay out of sexual relationships until she and the guys she sees are mature enough to handle it. I know it feels strange and wrong, but it is normal and anything you can do to help her become good at it, relieve the pressure while getting her to understand the pit falls of sex at her age with boys would be a desirable outcome I think. I think you need to not think like a Mom for awhile but like a horny teenager that has the knowlege and skills to be able to reach great heights of pleasure without...a boy.
So there is nothing weird about your statement or encouraging masterbation as a good alternative for sex and it is just fun and healthy (for both of you if you are still feeling bad about it). Far less "weird" than knowing she is having sex with boys.
It is something one can't just turn off.