How come it's excepted for teenage boys to masturbate but not teenage girls?

Um, it IS... Not sure who you are talking to or getting information from, but masturbation is a totally normal human activity for both men and women. It is simply a part of life and normal and expected and not a horrible thing or something to be ashamed of. There is no difference in this behavior for men or women other than the mechanics involved.
That is sexist. Both sexes enjoy masturbation and neither should be more or less acceptable.