If two unmarried Christians live together but remain sexually pure, is the living arrangement itself sinful or only unwise?

Assuming the context of the question is two unmarried Christians who are involved in a romantic relationship. If this is the case I do believe it is indeed sin, even if remaining sexually pure. We often enter into these situations with best of intentions, but those intentions often crumble, for all too often we have the “what ifs” in the back of our mind and push them aside saying, “I am stronger than that. My relationship with God is stronger than that.” We end up leaving a back door open for the possibility or opportunity for sin. The Bible gives warning about leaving yourself open to sin.

“So flee youthful passions and pursue righteousness, faith, love, and peace, along with those who call on the Lord from a pure heart.”
2 Timothy 2:22

“But put on the Lord Jesus Christ, and make no provision for the flesh, to gratify its desires.”
Romans‬ ‭13‬:‭14‬

It is also sin for us, if having the knowledge of what is good, to refuse or fail to do what is good.

“So whoever knows the right thing to do and fails to do it, for him it is sin.”
James‬ ‭4‬:‭17‬

If we intentionally put ourselves into situations, knowing that it creates an environment where sexual sin is possible or inevitable, that is sin. The same thing applies to unrestricted and unaccountable access to the internet.

My suggestion therefore is do not do this and get some accountability for yourself and your partner.

If any other situation is true, roommates with no romantic interest, affection or attraction, perhaps unwise for different reasons but I would not say it is sin. Family, like sister, mother or cousins, not sin. Staying with an old person, not sin.

Now I know there are aberrations, for these the same principles as above would then apply.