I believe in it as a legal institution, as well as a social one and an emotional one, where two people agree to spend their lives balancing the "me" and the "we," where they promise to be together until death as each other's friends and companions and lovers. All of those things are Biblical and good. I just don't think its enough.
With the couples I knew in college (and I suspect this is true of many others who get together young and spiritual), they were euphoric about how God was working in their lives and this new wonderful person he'd given them to share His work with. Many of them were also making the stressful but exciting transition to college and being away from home for the first time. As Gilbert points out, research suggests this also made them more vulnerable. They were naive, didn't have their identities firmly established (much less their careers or finances), and were just so idealistic. To me, that combination of newness, infatuation, and spiritual fervor involves WAY too much emotionality. Far too often, people get hung up on such feelings and what they must mean about the relationship. Its dangerous to make permanent decisions under those conditions but people do it all the time. Those feelings won't continue at that level for more than a couple years.
Don't get me wrong. I believe a couple should share spiritual beliefs and values. When people of differing faiths, or where one person has a faith tradition and the other doesn't, ask me how concerned they should be about this, I say this difference is very important. I let them know they'll always be using different spiritual vocabularies and frames of reference and that deciding how to raise any children they have will make those differences even more of an issue. I point out that family members and friends may think their relationship is wrong due to strong beliefs about how spouses should be of the same faith and concerns about how being with someone who is of a different belief system may impact the spiritual well being of their loved one. Many religions warn against this practice. I believe a common faith is extremely important. Still, I don't think a shared commitment to God is enough, in and of itself. Other factors need to be considered when deciding on something as serious as marriage.
Another concerning thing about these couples is that they were flying on the high brought about by the chemical changes in the brain related to early stage love aka infatuation. Most of these couples were only in their late teens and didn't have much dating experience. For that reason, I don't have a problem with setting some reasonable limits on physical intimacy. At a Christian university, such limits are to be expected. If a person's beliefs are that lust and too much physical involvement are wrong, they are going to struggle with guilt and suffer spiritually, and that sort of internal battle can cause a lot of difficulty for the person and the relationship. If they are very attracted to each other, so much of their relationship still becomes about sex and whether any behaviors they engage in are "wrong" or "sinful." I think this lead more than a few of these couples to want to hurry and get married.