Let’s say a boyfriend and girlfriend move in together and they have no intentions to have sexual relationships of any kind out side of kissing and cuddling. And also include that they are both very strong willed and faithful, and their relationship is modest. Is it still sinful for them to still live together?
Sin would exist insofar as the couple are putting themselves in danger of falling into serious temptation, and insofar as their arrangement would cause scandal.
Chastity is not simply a matter of being strong-willed and not having an intention to fall, but it is a matter of grace. Any young person who takes seriously their need to cooperate with grace, should take seriously the teaching of the Church, who has been established by Christ as our guide.
We shouldn’t flatter ourselves (or succumb to pride) by thinking we are beyond sexual temptation, because our souls will be in grave danger if we are so careless.
Having said that, many cohabitating couples may not have been Catholic (or any other type of Christian) when they started living together. Should they convert, and truly repent and live as brother and sister, and should they not run a risk of causing scandal, then they may certainly pursue marriage (if free to do so) despite the fact that they still live together.
Let’s not forget that the sins of cohabitation are fornication, the near occasion of sexual sins, and scandal. If all of these are avoided, and if a couple are in the process of getting married, then they will not be sinning if they continue to live together, at least not because they live under the same roof.
But a young couple who do not live together, and who are attempting to take their faith seriously, should under all circumstances be convinced to live apart until marriage.