That was my hesitation as well, Bear, but I also don’t believe there’s a clear line of delineation. If there was something that specifically stated that two unmarried people of opposite gender not live together under the same roof, I would understand. Otherwise, it seems the preconception in this is that sexual intercourse will occur between an unwed man and woman under the same roof, and I’m not sure if that’s always the case though it could lead to that.
If you had sex with her before her husband died you committed adultery. After her husband died when you first had sex with her (if you did) you became married to her.
The real problem with cohabitation for the Christian is that everyone assumes that if you are living with a person of the opposite sex then you are having sex with them. (Which I’d hazard is true more often than not)
So even if you are not sinning you are leading others to think that sin is OK (After all, if rarestia is getting laid every day I may as well too. He’s a Christian so it must be alright).
This is why 1 THES 5:22 tells us “Abstain from all appearance of evil.”
Our witness is important and must be protected.