It’s actually more easily understood when you evaluate what “springs” from the heart. A couple that engages in sexual activity prior to marriage most likely is going to live in a perpetual state of sin. They don’t *intend *to live separately, they *intend *to continue having sex, and they don’t intend to repent for it until perhaps someday they agree that its wrong and come to the church to get married. So in their hearts they excuse themselves and keep on doing what they’re doing. Perpetual state (a greater sin).
The person that has a “one night stand” can be deeply remorseful for that action and repent and come back into full communion with the Church and resolve to “sin no more”. This is no different then someone who has a problem with impurity, or perhaps some other grave sin. Falling short of God’s commands is something that most of us try to avoid but sometimes need to be forgiven. This forgiveness is asked for and given with a resolution to avoid the sin in the future. This is not possible in the scenerio of the couple living together and engaging in pre-marital sex, or a homosexual couple (also in a physical relationship), or a couple remarried with no anullment (outside the church) etc… These things are all the choice of the individual to remain in a perpetual state of sin, that’s why it’s completely different.