I dont think it is grown up or sexy. It makes the wearer look trashy. Yes, this is my opinion. I think you should listen to your parents as I am certain they are the ones that are providing for your very existence. You can do what you want when you move out on your own.
I can see you are a very polite young lady so please know I am saying this with the up most respect for you and your family. But in my opinion you CANT convince your parents if they are good parents. :0) They say no for a reason other then to make your life a living hell! :0)