When I was 19, I went to work in London (in 1985). I grew up in Hampshire, attended a comp, which was overwhelmingly white, (can't remember one non white child at school), middle class, and nominally Christian.
I worked in an office where I came across religions, creeds, skin colours and cultures that I had never encountered in reality before. Had we had RE to O level as opposed to CSE, then i might have known about some of these. My colleagues were unfailingly polite and good natured to me in the face of my ignorance about them.
RE is not about proselytizing, the best RE teachers I know are atheists; it is about equipping students for the wider world. Not every child grows up in a metropolitan borough in a cultural melting pot. Many grow up in areas that are white, and that don't have any other religion than Christianity and Paganism. If we didn't teach these children about other religions, we would be doing them a disservice.
I also taught about euthanaisa, abortion, crime and punishment, war and peace, conflict resolution, community cohesion, religious beliefs or not, wealth and poverty, religion in the media, philosophy of religion, religion and the environment, marriage and the family, contraception, sexuality (I still laugh when I remember making a whole row of Year 10 boys blush). There's a wide variety of stuff that gets addressed in RE lessons, and that students can't, or don't want to, raise with their parents.