When you're married, you are free to have sex emotionally and physically. You know each other's boundaries, and sexual history (health wise, anyway). And you are prepared to have kids when not using protection. It's just better when you're married, because it is with the one you love, and there most likely will not be any regrets afterwards. Most people who have sex before marriage feel regret afterwards, and are down emotionally because either the person left them, or they just weren't "the one".