You are both in a state of flux right now, meaning your futures are anything but secure. You're still learning, you're still being educated, you have little practical experience living on your own or with another human, you have no finances for a family (I'm guessing, unless you plan on making the 800K it takes to raise a child). You are in the midst of meeting all kinds of new people from all walks of life; you have no idea who will turn your head. More than that, the human brain isn't even fully developed until about age 26.
You cannot understand this now, and you're not supposed to, but no matter how mature you are for your age, you're not even close to how mature you'll be in five years. Trust us on this. So the choice is yours: Marry impulsively and secretly (no, it's not a sin but it's not mature) or wait a few years, both have solid educational foundations and economically sound futures, get out and experience the world a little and then have the wedding you've dreamed of.
If it's right, it'll wait. If it's not, then thank God you waited. Good luck.