You are obviously uneducated. There is risk in sexual intercourse which you either do not know about or simply refuse to acknowledge. There is first the possibility that any protection one may use may fail and yield an unwanted child. There is also the possibility of sexually transmitted infections spreading while either known or unknown by the carrier. Many of these infections are incurable, and can yield infertility and death.
I suggest you do not try to say "that never happens to anyone if they're careful" because that is simply ignorant. These infections DO get transferred from person to person, and while the majority of people may not have one, when you commit sexual intercourse you are exposing yourself to potential diseases that that person may carry through any number of his or her previous sexual partners. In common social vernacular, it could be said that "when one has sex, one is also having sex with every single one of their partner's previous sexual partners."
Edit: I think it important to also mention potential emotional damage. While irrational, it occurs and is a great cause for dismay which lasts more than the brief time period in which the act is performed.