the ones that have, have done so after leaving disney, as a way to get rid of their child friendly image.
but whilst with disney it's really not a good idea.
i remember reading an interview by one of the jonas brothers about the time vanessa hudgens' nudes leaked during the highschool musical era, and they remember being scared shitless about it and being in the building when she had her meeting with execs worried about her almost as if she had just received the death penalty. and after that incident, disney started pushing the purity ring bullshit onto their teen stars. and those teen stars were advertising it left and right in all their interviews to let it be clear to the public they were virgins and weren't having sex until they got married.
you don't want to fuck with disney's image, or they'll fuck you tenfold