Numerous studies consistently show that cohabitors have much higher levels of sexual infidelity than married couples. According to research done at the University of California-Irvine, "The odds of a recent infidelity were more than twice as high for cohabitors than for married persons." This held true even when researchers controlled for issues such as permissive values about extramarital sexuality. They concluded that the "commitment mechanisms" of marriage were the likely reasons for the difference.
The highly reputable National Sex Survey found an even greater divide, reporting that live-in boyfriends are nearly four times more likely than husbands to cheat in the past year. And while women are generally more faithful, cohabiting women are eight times more likely than wives to cheat. It seems that if you want to give someone the experience of sexual opportunism with other partners, cohabitation is what you are looking for. But if a faithful partner is your desire, then, cohabitation is not your thing.
Glenn T. Stanton, The Ring Makes All the Difference, pg.47