Knowing that you mean "why wait to marry", I'm gonna submit something that may be hard for people to do, which they may indeed not do, but is still good medicine.
They would be wise to wait for this reason (I say wise because I'm not going so far as to say right or wrong but wise, to which there is a difference): in our society the young are growing up too fast but maturing slower than ever. They don't know what is real when it comes to the term "passionately in love" and they don't have a clue of what a true relationship really is. They are taking their cues from the likes of Snooki off of "Jersey Shore" and from their own homes that are filled with dysfunction, etc.... From the time of the generations around me (I'm 37) and on it gets progressively worse. AS A BELIEVER, if I had gotten married every time I "JUST KNEW" I was passionately in love I could have as many wives as Solomon. That was in the 90's. It is 10 fold worse now. I'm not saying that there are not young adults who can absolutely know they are in love and "meant to be" with that significant other. But that is generally the exception to the rule rather than the norm. Plus, saying such a thing as "we just knew we were suppose to be together" was made of more stern stuff when our grandfathers and great-grandfathers said it than when today's microwaveable generations say it. They're changing their minds at the same pace as changing underwear. So on the whole, I would put forth that if indeed they are passionately in love and know such a thing is "meant to be", they would be wise to see a thing grow and blossom. Time will show them and teach them much that will help ratify their "feelings" and indeed make them even stronger as a "meant to be" couple!
*sigh.....* We just can't wait for anything these days. It truly is sad that we cannot delay gratifications which would lead to better, more enriching outcomes. We can't save up a little money to pay for that item so we borrow ourselves into debt and bankruptcy because we had to have it NOW! The same is true for relationships, which in our society are just as similarly bankrupt. Such people as you suggest in your post would show maturity, wisdom, and would be well-advised to wait. There is only the rare exception to the rule. With such a covenant that God says shouldn't be undone, there is close to nothing to be gained by rashness and much to profit from delay. If humanity had the fortitude for such, I've no doubt the divorce rate would plummet (at least among believers). However, in these last days, I've little hope for such.
Elansing, what you did was right, scriptural and the godly thing to do. Yes, the painfulness and difficulty of doing so breeds some doubt about the decision. But some will say no. Some will only follow because they are fed and will show their heart as they walk away when things stop meeting their physical, fleshly needs. And a broken-hearted Jesus, watching the good but un-surrendered rich, young ruler for example, will allow them to do so.