A recent study published in the Journal of Marriage and the Family from the Univ of CA at Irvine reports that cohabitation has little effect on a couples expectations of their partners fidelity (94% expect sexual exclusivity compared to 99% of married couples), but that it markedly effects the likelihood that they themselves will remain true.
“The odds of recent infidelity were more than twice as high for cohabitors than for married persons”, the researchers calculated. They also found that couples who have been “living together before marriage raised the net odds of marital infidelity by 39%”.
[The Family in America, 7/00; J.Treas & D Giesen, “Sexual Infidelity Among Married and Cohabiting Americans,” Journal of Marriage and the Family 62, 2000:48-60;Abstinence Network, Fall 00]