My protestant brothers and sisters called it giving an “Appearance of Evil.” It causes others to stumble, and despite your good intentions, they will never believe that you didn’t “give in” to temptation. And, seriously, would you want them to think that you were weak like that the entire rest of your relationship with this person, especially if you were to marry? And finally, it’s best to live together with the opposite sex only under the safety of a sacramental marriage and all the divine graces that come with it. It gives you courage, strength, and forbearance that you would not have had without it.
God bless,
-Amy