Well, the more people have sex, the more likely they are to catch sexually transmitted infections (STIs) or sexually transmitted diseases. If we make the commitment to one partner, then we know what the risks are. And if we know where they've been, as well as where we've been, the risks are minimal.
According to God, one of the purposes of marriage is for procreation because the institution of marriage provides a stable basis for a loving and devoted family. Many women wouldn't end up getting pregnant before they're ready and having abortion, keeping the child and having hardships, or leaving the child up for adoption. Not only does the parents future get torn, but also the child's. Many children don't even know who their parents are.