Without mods, relationship culling is very bad in this game. I have seen it totally destroy a boyfriend-girlfriend couple's relationship, just because they don't live in the same home. If you are interested in stopping it, there is the MCCC mod that will stop this, but once in you'll need to work on the settings. Once the settings are the way you like, it will take care of all your game saves. Here is a link to it in case you don't know about it.
https://deaderpool-mccc.com/#/releases
They also have a very good support over there with live chat for any questions you have.
