Although it is permitted under the first amendment, rap music is highly offensive and it should not be promoted in mainstream society. It promotes negative racial stereotypes, and it advocates objectification of and violence against women. Also, it's not good music. No one in their right mind would listen to rap music.
There are all kinds of music that promotes false stereotypes but we do not censor it. If we start censoring Gangsta rap then we would have to start censoring virtually all music. This is America and for the most part we have free speech. Why stop at Gangsta rap? If we adopt the censoring mentality we would have to start censoring books and movies too!