Depends on the people involved in each act.
Are we talkign about secular, atheististic, or even liberal Protestants here? Or are we talking about Catholics who know the teachings?
**If they’re ignorant then both sins are lesser, and may not be fully mortal and full knowledge would be questionable. **However, if the couple in the relationship are Catholic then they are committing a worse sin - if they truly loved each other they’d wait till they were married and then have sex. Instead they’re continunally damaging each other’s souls and each other’s chances to get into heaven, all the while committing the sins of scandal, probably contraception, and are willfully flouting God’s law, justifying it to themselves as “okay cos we love each other xxooxx”. Whereas the one night stand scenario involves individuals of an obviously lesser moral standing, probably a good lot of booze and the possibility of repetenance when they sober up and regret their action.
So I submit that those in the relationship are in worse trouble than the people comitting a random, one off act of shaggery.