I think because sex before marriage focuses on the pleasure of sex not on the purpose of sex. Sex's purpose is the survival of the human race, with the pleasure as an added bonus. If we focus on the pleasure and forget the purpose, things go downhill. Like food, which its purpose is to survive with the pleasure of eating as an added bonus. If we focus on the pleasure of eating, we will eat what tastes good (like pork, junk food, etc.) and hurt ourselves. We eat to live, not live to eat. Similarly, if we focus on the pleasure of sex, we will have sex when it feels good (outside marriage, anal and oral sex). We will be living for sex, not having sex to live. The pleasure will control us rather than we controlling it and we will be focusing on it and not on the purpose of sex, which is the survival of human race.