I actually think that hopeful4 makes a good point. I don't aim to be judgemental, here...because I'm not....and, not on a moral basis, either....
But, simply because it is protective for the children...in case things ever go South. (I'm talking about kids under 18yrs.). And, it does protect the parent, to a certain extent, also...……….
since blended families can be tricky, at best, to navigate....it eliminates additional life problems for the parent to have to cope with...lol...…
I didn't live with anyone until marriage was imminent...(a few weeks)….However...I did not deny my own social dating life.....and, the kids did get to know anyone that I was dating, long term. But the kids did not have to adjust to someone else living in our nest....they seemed to understand that there was no parent figure that could possibly "leave" them, in the future.....ever.....
So, if things do go south...it is so much easier to break up...in so many practical ways...