[litany_of_lies]

What is the harm in waiting at least 2 years until the graduate?

Maybe none, maybe a lot.

I'm assuming they're engaged, "sure," and "prepared."

The longer they're engaged, the longer the "danger period" for premarital sex. If they marry, they're under the sacramental umbrella. I think couples have the flexibility to decide if they'll be able to hold out, and I wouldn't want to criticize them if they prayerfully conclude they can't (Paul's magic word, subject to interpretation).

I'd leave it to their discretion, as long as they are open to the possibility of having a child while practicing pregnancy-chance-minimizing NFP during what I still think could be classified as a "qualifying serious situation" (no natural method shuts the door entirely) until they graduate. I think we can give them the benefit of the doubt without accusing them of being selfish if they choose to marry ASAP. You could send the selfishness accusation in the other direction too--"you're sure, you're prepared, and you're not going to take responsibility for each other right away?"

[Hermann the Cherusker]

So then, the question is why would someone put themselves in the occasion of sin of "going-steady" and "getting engaged" in College, when there is no realitic prospect for forming a Christian Marriage at that time?

You can't stop people from falling in love, but people can put their focus elsewhere, and avoid looking for love at the wrong times.