Why should we wait until we're married?

i love this girl and she loves me. we're gonna get married one day. i know what your thinking. "i'm too young to know what love is; its just puppy love; blah blah" well, i love her. and i know it. we both feel ready that we want to experience the ultimate expression of love together ( :D). but ive got some doubts. the only way we can do it is to sneak off to a motel. i dont really want our first time to be like that.... but I want to have with her. really bad. she feels the same. but why should we wait? im gonna wear a condom. can someone give us a reason why not to? besides, i'm a hormonally-imbalanced(hyperthyroidism), viciously and incredibly lustful 17-year-old boy. i can't wait another second. :D why is it frowned about anyway? what's so horrible about premarital ?