Not listening to parents is bad karma?

This is something that I keep hearing from my grandmother that annoys me so much. Although I am of age (26 to be precise), I am still expected to listen to almost every word that my parents say (at least according to my grandmother). Not to bring personal matters to the site, but I keep arguing that not listening to parents should not be considered ill-treatment of them. They expect me to wear nice clothes, but I don't. (I prefer wearing the same clothes. Who has time for fashion?) They expect me to marry the one they want. They always say that as a Buddhist I should be very obedient. These days when you turn on the TV to listen to some Buddhist advice, monks most of the time talk about parent-child relationships and go on about how children should always listen to their parents. I am not saying my parents ask me to do anything bad, but as children we know (sometimes, not always) what is best for us. But when we do what we want all hell breaks loose. Daddy shouting, Mommy crying, and Granny shouting that my bad karma will eventually give me disobedient children just like me. Jokes aside, I want a serious explanation when it comes to parent-child relationships. I am quite aware of the Sigalovada Sutra, but when we do something we want and parents get hurt, is it always bad kamma? It can't be, right? Otherwise the amount of bad karma Prince Siddhartha would have gained after all the pain his father went through... Please share your thoughts.