When any relationship has no real commitment before intimacy goes too far (on any level - emotional, mental, physical, etc.), then the persons in the relationship, no matter their good intentions, are playing Russian Roulette with the relationship.
The analogy that is most commonly used is that cohabitation is like taking a car on a "test drive". The problem is that when we use people as objects, it is the worst thing we can do in a relationship. Pope John Paul II said the opposite of true love is use. This is because we make them less than human when we use them.
When a couple cohabitates they are using each other in every respect, whether it be for sex, companionship, intimacy, good feelings, etc. - because true love is wanting the best of someone regardless of the cost to yourself. Putting a relationship in such a "danger zone" is never loving. It basically is saying to the other person (or more than likely to each other) - I see you as useful to me at this time and therefore I am willing to take a risk in hurting you physically (pregnancy, disease, etc), emotionally, spiritually and the future of our relationship and others. If there was true love, then we would have a binding commitment to the future of the relationship - marriage - before living together.
Thus, cohabitation can never be about love. This is the reason that couples who cohabitate before marriage divorce almost double of non-cohabiting couples.
Marriage should be a permanent state - for Christians it is a covenantal and sacramental bond that is irrevocable. This permanence along with faithfulness offers a safe environment for real sacrificial love to grow. When a relationship can be changed like a shirt, love can't grow to it's fulfillment. To make yourself a true gift to someone is the point of marriage. When you cohabitate, you are only able to give a partial gift - which points the relationship down a dead-end street. A partial commitment is no commitment. Another big thing to consider for Catholics is it can be argued a cohabiting couple may not be able to validly marry. This is a very serious consideration.
Reasons not to cohabitate are numerous and I have many of them above. Here are some further stats:
- One-sixth of cohabiting couples stay together for only three years.
- One in ten survives five or more years.
- The rate of divorce among those who cohabit prior to marriage is nearly double (39 percent vs. 21 percent) that of couples who marry without prior cohabitation.
- Cohabitors who never marry have 78 percent less wealth than the continuously married.
- Compared to children of married biological parents, children age 12-17 with cohabiting parents are six times more likely to exhibit emotional and behavioral problems.