Based on the question you asked, I am concerned that you have an incorrect, secular understanding of marriage.
When a man and a woman make life-long vows in front of witnesses, it involves not two parties but three parties:
- the single man
- the single woman, and
- God, giving them the sacramental bonding grace that only the Lord can provide
If you said your vows privately to each other and unofficially married one another, you would not be married, even if you thought you were married.
Interestingly enough, this is one instant where the state would agree with the Church!
Making your martial vows in an unofficial, private way, is like driving a car without having first taken driver education classes and being certified by the state to drive.
That way if we did have sex, it would be in marriage and wouldn't be sinful.
Yes, it would sinful. Concupiscence or a desire for the flesh, according to the Catechism, is a life-time struggle that we all have to endure, whether we are married, or not.
As my mother drilled into me, Christian marriage and having children is a lifetime commitment. The Church wants to encourage Christian marriage and family life but in a responsible manner (with a level head on both spouses) as to the joys, sufferings, and glories of the married life that awaits them.
No matter how long you have been dating, if you are not even old enough to marry, I would suggest that you continue to develop your friendships.
That's my two cents.
My colleagues may have additional advice.