Being "shacked up" doesn't mean anything differently than being married. Most marriages don't last and cost a considerable amount of money to get out of it whereas being "shacked up", if it doesn't work you can just walk away. The plus side of "shacking up" is, I would think, you'd have to work harder for it to be successful - you take your spouse for granted when you're married ( generally speaking) and that leads to someone being selfish and that leads to the end. My older, wiser eyes would've preferred "shacking up". 