While I understand the point of view, and agree that women shouldn’t be pushed into the roll of being all “nicey-nice”, I disagree in putting forward the idea that such negative emotions should be dealt with at all in this way.
I guess it depends on your idea of “letting it out”, but giving focus to your negative emotions only cements them more in your mind, and allows them to take more control of you.
I’d suggest that instead of viewing the equality argument as “women should be allowed to be angry”, it should be viewed as “ we all should try to be more calm”.
Repression of course is not a good thing, but there are more ways of dealing with negative emotions than letting them out or keeping them in. A different frame of mind can be developed which allows for all emotions to be viewed in a more objective light, which can remove a lot of their power they have over our mental states.
We all struggle with anger and hate and pain, but being equally openly angry and hateful and pained won’t do much good for the progression of our society.
So, if you’ve spent your life in the role of a “nicey-nice” woman or girl, perhaps a better way to flip is to just start being that way for yourself, instead of for some men (or other women) in your life. Act as a positive beacon for others to look up to. Cultivate inner calm to go with the already practiced exterior calm, and let that help you to make others more calm.