Cohabitation is not marriage. Children do about as well as they do in single parent households even adjusting for income and education. Access to two adults is not the same as parents. If you remove sex from the justification for cohabitation, it will not happen, regardless of what the couple (usually the woman) uses. Some countries have institutionalized cohabitation--don't live with your girlfriend in Canada--the state will come after you after you've moved back to the U.S. unless you've agreed to a contract that denies your commitment.
"For most of the latter-half of the twentieth century, divorce posed the greatest threat to child well-being and the institution of marriage. Today, that is not the case. New research—made available for the first time in Why Marriage Matters—shows that the rise of cohabiting households with children is the largest unrecognized threat to the quality and stability of children's lives in today's families."
"Cohabitation is playing a growing role in children's lives. Children are now more likely to be exposed to a cohabiting union than to a parental divorce. The report indicates that 24% of kids born to married parents will see their parents divorce or separate by age 12, while 42% of kids will experience a parental cohabitation by age 12."