The OP is right to try to reason beyond mere rules and slogans to the principles that underly them. That’s an excellent practice.
Obviously fornication is a grave sin. But WHY is it a grave sin? By definition, sins are things that reduce our capacity to give and receive love. On the larger scheme of things, a major that fornication is wrong is because it dilutes the precious and unique bond that God intended for a husband and wife to share only with one another. It weakens the effect and subsequently God’s design for the marriage bond.
So doesn’t that support the OP’s supposition that premarital sex between two people that later marry is really no big deal?
No. You simply AREN’T married until you are. If you are intimate with someone to whom you have not been joined together by God, then you are saying with your ACTIONS “I am willing to have sex with someone not my spouse.” Wiggle all you want about intentions, the fact remains that right now, she is NOT your spouse. That message remains and festers, even if you do marry. Plus, intentions change, surprises happen, minds change. You aren’t married until you are. Jumping the gun will gravely wound you.
If you’re THAT sure you’re going to get married, just go ahead and GET married. If there are real reasons not to yet, then there are real reasons NOT to yet! Telling the truth with the rest of your body is as important as telling the truth with your tongue.