The act of usually a man and woman living together having sex and playing "house", pretending to be married but without marriage benefits. Cohabitation is never a good idea for a couple who is possibly considering marriage. If a marriage occurs and is meant to work out, then cohabitation is not necessary and could even lead to a break up--it usually does. Cohabitation is also not necessary because you should be able to know enough about your partner ahead of time before marrying them.
see: living in sin
Sociologists say that cohabitation among a couple before marriage make them more than twice as likely to end up divorced. It is a falacy to believe that cohabitation behooves a couple.