Sex becomes bad when you are not matured enough to handle what comes with it, it can ruin ones life and change the order of one's destiny if you don't exercise caution. It could lead to unwanted pregnancy, and other life changing infections or diseases, and this is not for Kids. I think this is why some religious leaders preach it bad because under the context of Christianity, sex is right only during marriage with a right partner, not only for pleasure but for procreation as well.