It is the trend these days to window-shop before you buy. And of course I am referring to people of all ages living and sleeping together outside the public vows of marriage. This is especially common with the twentysomethings, but it is increasingly common with all ages - even teenagers (think Teen Mom TV shows). Now someone will say, “Well, duh. Where have you been since the sixties?” I write this now, in the year of our Lord 2013, knowing full well that this is nothing new. There has not been some recent epidemic of co-habitation that has sparked these brief comments. However, a recent study has come out saying that nearly half (48%) of all “first unions” have nothing to do with a white dress and altar. Furthermore, the report shows how this is a reality on the rise. 
That is a staggering number. Like a bucket of ice water down the back of your shirt, it makes you stop and take notice. As Christians then, what do we believe? Is this healthy? Is this wise? What behaviors do we encourage, and which do we preach against?
Since this is the page for being quite candid, co-habitation is sin. Living together in a sexual union outside the vows of marriage is what the Bible calls (and what 1,967 years of Church History has called) fornication. In other words it's a bad thing for people to do. We are called to flee youthful lusts, and to put away all sorts of sexual impurity. “But I love my girlfriend as if she were my wife. I don’t cheat on her at all. Promise. Isn’t that being faithful as Jesus loved the Church?”
I do not want to answer that question by taking you on a trip down the statistics aisle, showing how co-habitation leads to other painful things like divorce. They are available online for you to find if you are truly asking rather than simply shifting in your seat. My purpose here is to simply tell you why we believe and preach co-habitation to be sin. 
God created marriage to be something set apart and different from other relationships. It was created in the Garden between one man and one woman. This was something that required God’s consecration (big word meaning ‘to declare something sacred or holy’). The vows, in the middle of the wedding ceremony, are the actualization of this. The man is telling the woman that he promises to hold this relationship as something sacred. It is not just special, but sacred. The man is committing (and vice versa) to honor his promises of faithful fidelity to this one woman. Living together outside of the solemn giving of vows is not a sacred act. Rather it is treating as common that which God says is holy.
God also created marriage to be a something in the context of (or alongside of) something else. Leaving father and mother, the husband and wife become their own something. Therefore, vows must be done publicly with witnesses. Wedding guests are there to watch this mysterious setting-apart-as-a-unique-something happen. The two come as two, and leave as one. Be it in a church, a court house, or by the lake, the mystery of marriage descends on the couple, changing them forever. Living together outside of the public ceremony of a wedding does not make you one flesh. Even having sex does not make you one flesh. (Which is why sex outside of marriage is so dangerous - where there is the beauty and unity of the sexual experience with two ‘fleshes’, the order of creation is being torn apart. Only pain can come of it.) You are one flesh with your wife because of what the wedding accomplishes, by the mystery of God. The wedding night is an expression of the wedding itself, and not the other way around.
Marriage is an institution; it is not a preference. Marriage is the enactment of higher and eternal realities; it is not an outdated mode of living. There are many more issues that relate to this: the authority of fathers, the protection of daughters, and the honor women are due. But these must wait. For now, “What God has joined together, let no man tear apart.” What God has made sacred (with vows) and public (ceremony), let no man tear apart, either by a flat-out rejection of marriage (divorce) or by simply ignoring its importance (cohabitation). 
Again - the Gospel of Jesus Christ stands as both the standard for living and the means to achieve it. But it can only be done in Jesus and through the Spirit and to the glory of God the Father Almighty, maker of Heaven and Earth.