What does a woman mean when she answers "I don't know" to many questions?

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Many times, when I go out with my girlfriend I has her questions like 'what do you want to do this evening', what do you feel like eating', where do you want to go tomorrow' and such. She usually answers 'I don't know'. Every time she says that I get more and more aggravated. I wonder if it is really possible that she doesn't know (guy's take on that response) or if she really means something else. If the case is the latter, what could she mean? I do come up with things to do, but then many times she will say some stuff that makes it clear that the activity I chose or the places where we went aren't quite what she expected or what she wanted to do. That's why I ask her for her input every time. I am lost. I need help!!! Are women really speaking venusian while we are speaking martian?