Does what other people think matter?
The Bible itself tells us, “Abstain from all appearance of evil” (1 Thessalonians 5:22, KJV). Jesus said, “Let your light so shine before men, that they may see your good works, and glorify your Father which is in heaven” (Matthew 5:16, KJV). I do not see that this arrangement contributes to the glory of God. Your friend may thumb his nose at the world, charging that those who challenge him have “depraved” minds, but the fact is that essentially everyone— Christian and non-Christian alike—will assume that they are having sexual relations. In my view, the damage to his reputation is not worth whatever benefit he may think is there. Further, by maintaining this arrangement, he is damaging his girlfriend’s reputation, for people will assume the same of her that they do of him. He should take godly initiative in this matter to bring it to an end for both their sakes—and especially so if he regards her as his girl-friend. He should be seeking to guard her reputation even more than his own.
Here are a couple statements from Mrs. White that might apply.
It is not enough for you to avoid the appearance of evil; you must go farther than this; you must “learn to do well.” You must represent Christ to the world. It must be your daily study how you can learn to work the works of God. His followers are to be living epistles, “known and read of all men” (Messages to Young People, 348). [What are people “reading” in the epistle of this man’s life now?]
It is important that children and youth should be trained to guard their words and deeds; for their course of action causes sunshine or shadow, not only in their own home, but also with all with whom they come in contact. But before the youth can be careful and thoughtful and refrain from every appearance of evil, they must have that wisdom which comes from above, and the strength which Jesus alone can impart (Messages to Young People, 345).