See John 19:27 Then He said to the disciple, “Behold your mother!” And from that hour that disciple took her to his own home.
See how Jesus made a home in Jerusalem for His mother so she would not have to go back to her family home. She could continue living with those people with whom her heart resided.
I believe Mt was adressing a boyfriend and girlfriend living together which can be scandalous unless there is a good reason for doing so as discerned through prayer and consultation of your pastor (i.e. a fiance is sick, etc.) Sure, there are instances when unmarried men and women can live together. But no one is going to say that Mary living in John’s home is the same as a boyfriend and giflfriend arrangement.
Cohabitation generally implies sex without marriage (thought not always, and the two need to discern to make sure they live chastely.) If the individuals are having sex, they must be counseled to stop, and as related to encouraging them to get married right away, I would be cautious if they have been having sex. Rushing to get married should not be based upon the sin of having sex outside of marriage. Often times, the couple needs to slow down to discern the full meaning of the vows they will take and really start to learn their potential future spouse without the blinding light of sex. Even studies point out that people who live together before getting married have a higher likelihood of divorce and seperation down the line. So, I would counsel for prayer and discernment with the aid of a solid priest.
And if they are living together so one can chastely care for the other, God will grant them the Graces to remain chaste and serve each other until one day Lord willing they will be able to share in the Sacrament of Marriage. Thanks and God Bless.