Got busy, here ya go
I completely disagree with the aforementioned interpretation of Paul's instructions and his alleged oversight, but I will address that separately. It appears the secular reasons are more in demand at the moment, so without further ado:
It is, statistically speaking, bad for your marriage.
I checked into a couple of articles and found that couples who live together prior to being married (or engaged, depending on which article) are up to 40% more likely to be divorced down the road. While I found several articles on this topic, the actual research that was being cited was harder to track down. Most of the research I located required a purchase to obtain the full document, but I do have some snippets:
Links to actual research:
http://www.vifamily.ca/library/cft/cohabitation.html#Do
http://psycnet.apa.org/journals/fam/23/1/107/
A couple articles:
Discovery Health "Does living together before marriage lead to divorce?"
3 Reasons Why Researchers Say Living Together Before Marriage is Risky - iVillage
Living Together Before Marriage Letter #1
Quite simply, divorce rates are higher, depression rates are higher, relationships with other family members tend to be more strained, sexual activity (if there is any) is less satisfactory, and most ironically, 20% of cohabiting couples are still unmarried after 5-7 years.
Of course, that is all googled stuff, which is cheating I guess. For my own personal thoughts on the matter:
1. Wrong perspectives
Anyone who goes into cohabitation with this 'trial run' mindset is doing it for the wrong reasons, or at least from my perspective. Bomby referenced this concept on the first page; basically, seeing if you are compatible. My problem with this is that you should not have to live together to figure that out. If your relationship is so unsure that you feel the need to live together just to test it, your relationship is already weak.
2. More drama
What if you break up? Breakups can be nasty, everyone knows that. Broken hearts and drama, lots of crying, etc. On the other hand, some couples simply decide to end it and go their own separate ways without incident. Some even remain friends. But if you live together, now you have the added step of having to move out, finding a new place to live, sorting out the belongings, etc. An average breakup could easily go from a typical separation to a huge dramatic argument once all these extra factors are considered.
3. Marriage of convenience
Along the lines of the previous point, even if the relationship is not going well, some couples inevitably end up staying together simply because they are already together. In this way, they can avoid the extra hassle. Obviously, a marriage founded in such a way is not getting off on the right foot.
4. Added financial tension
This is kind of a no-brainer, but unless the couple have both agreed to joint-accounts (before marriage??? not likely), then there will be discussions of who has to pay for what. Ideally, this would be a simple and easy issue... but we do not live in an ideal world and I see potential for conflict here. It is not guaranteed, but it is certainly a possibility. Quick example: the couple splits the cost of an LCD tv and it eventually breaks. They both paid for it, but who pays to fix it? Since they both own it, it could be argued that they split the cost of repairs 50/50... but what if 80% of the usage was by the man? Is he more responsible than her? After all, he uses it more. This is just one possible scenario out of hundreds.
While a couple of other thoughts crossed my mind, it is 1am and I have to work tomorrow, so I end it there. These are posted to answer ZG's query, not spark a debate. I am too busy as of late to participate in an ongoing argument anyway.
So there you go my friend, that is how my mind ticks. :)