Developing the relationship first lets you find out if this is someone who you really love. Having sex is no guarantee you'll stay together. Some even get pregnant, thinking that a baby will cement the bond. Then he leaves anyway. Most men will find a woman more desirable if she chooses to wait.
Men know the difference between love and lust.
He may be just using you for sex. After you've had sex with someone, breaking up hurts twice as bad. When you trust someone completely and then that person walks away, you may feel used in a way you never have before. If you give your body to the wrong person, it won't take a baby or a disease to make you feel terrible. So think about waiting.