Will rationalizations never cease? First, we are to avoid the appearance of evil. Second, it is a serious sin to place onself in the proximal occasion of sin. If a couple tried to convince someone in the world that they were sleeping in the same room and yet being chaste, well… let’s get real! What is worse is that most people wouldn’t even ask and unless the couple went around with T-shirts saying “We are living celibately together” everyone would presume that this Christian (assumption) couple were hypocrites or not living in accordance with the teachings of the Church or Scripture, etc. This could give scandal to others, cause others to think it’s okay to do the same, and otherwise give a bad example.
I know this first hand because my nephew (who had always been a devout Catholic) moved into his parent’s house with his girlfriend turned fiance. I could strangle my sister for allowing it! He claimed that they were living a chaste life. But…one thing led to another and before you know it they moved into an apartment together before getting married. He later admitted to me that they could not resist the temptation. Sadly, during that time my nephew’s faith suffered and now that he’s married, he doesn’t even attend Mass regularly any more. His wife is not Catholic and not even a practicing Christian, so what kind of example is that to her?
The effect on his younger brothers, whom he was discipling, was devastating.
So, at the least it is foolish, at worst it is a cause of scandal in others and placing oneself and ones loved one in the proximal occasion of sin - thus, it is a serious sin in itself and a very unloving thing to do to one’s future spouse.