For starters they are putting themselves in a near occasion of sin. Human beings can be weak, even with the best of intentions (Mt 26:41). However there are a number of issues that social scientists have found with couples who cohabitate before marriage:
From the USCCB's document on cohabitation:
- On average, marriage preceded by cohabitation is 46% more likely to end in divorce.
- The risk is greatest for "serial" cohabitators who have had multiple relationships. Some studies indicate that those who live together with definite plans for marriage are at minimal risk; however, there are no positive effects from cohabiting.
- Social scientists have tried to determine whether some of the risk is due to the selection effect, i.e., that people who cohabit are already those who are more likely to divorce. While research shows the selection influence, most social scientists emphasize the causal effect, that is, cohabitation itself increases the chance of future marital problems and divorce.
- Cohabitation usually favors one partner over the other. Studies find that cohabitors are unequally committed. Often, the more committed partner is willing to put up with poor communication, unequal treatment, insecurity and abuse. Typically, women are more vulnerable, since they tend to be more committed.