We hear it in the car. I don't really like it but oh well. My DD 11, asked my why, I told her about the line about using "jack" to brush you teeth or gargle with. She didn't even know what it meant she just shrugged her shoulders.
I guess they are just listening to the beat of the song and the music it self and no thinking about the words. That being said, I probably wouldn't down load it for her because I don't like it. I do have to admit that my kids love Ima Be by the Blackeyed Peas and we don't have a clean version of it, They listen to it here but I won't put it on their I-touches for everyone else to hear.
To the OP, I think it is great that you are standing your ground. Parents shouldn't have to worry about being the kids friend or being the cool parents. You need to do what you,the mom, think is appropriate.