If you had an actual preference that you did not want disappointed, it would have been better to say that first. If I don't want DH to make hot dogs for dinner, I don't answer "Well, you're the cook and you can do what you like." He'll usually still press me for input, but I've already categorically opted out of desire for a particular outcome, and I have only myself to blame if he then makes hotdogs for dinner.
The difference is, the initial request is for my strong preference. If I don't have a preference, he'll probably still insist on getting me to suggest a couple things, as part of the decision-making process. Even if I don't have one thing I really *want* for dinner, I still might have suggestions for what he might like to make. I think what you're seeing as her insisting on your preference/opinion and then disregarding it, she was probably seeing as gathering your input, after you had already denied having a preference, then making her own decsion.
It's perfectly allowable to say, "It's your hair honey, but I think I'd really like it best if you just got it trimmed, rather than cut much shorter," or even: "You know I really prefer your hair when it's long, it looks so beautiful that way."