Let's not forget that this situation is not exclusive to young adults. Middle aged and older adults are also increasingly cohabiting for a variety of reasons, such as:
-marrying would cause a significant problem among one or both of the couple's child(ren);
-marrying would affect their Social Security or retirement benefits (naturally, I'm only speaking of the US); or
-marrying would require new legal burdens such as updating wills, real estate deeds, etc.
So we cannot just assume that this problem is present only among the young. I used to regularly visit a home for older adults, and in talking with the residents I really think just as much if not more is going on there than in a dorm.
In the past, I have had to recommend that if the temptation is too great, maybe the couple should consider not living together, but even then that's not a guarantee of non-intimate conduct between the two. Ultimately it resides with what is important to the couple, and where they draw the line in their own lives. We can only pray that their behavior leads to their salvation, and that the Lord have mercy on us all.