I switched households for a while and redecorated their house. Didn't even run the game to play the family, just redecorated then switched back to my original family. I noticed something was wrong when one of my married sims had the Friendly Introduction for her husband. When I checked the relationships none of my sims were married or even friends - all strangers! So, I got them all reconnected with MC.

Then I continued playing but started to get notices that so and so found my sim attractive and wanted to get to know her better. But, I realized something was very wrong when I got a notice that one of my non played Married town sims was having a romance with someone other than his wife. When I checked MC > Focus > Married it showed NOBODY in town, except my household, was married!

Its going to take me a month of Sunday's to get my whole town married again with MC AFTER I figure out who belongs to who. I have no idea why my whole town separated and became strangers to each other after I did a brief switch of households. Any suggestions?

OH crap! Yikes...I hope someone can help you. You do have a save file from when everything was okay don't you? Do you think a mod or something else did this?

littlelambsy - Forgive me for jumping to some hasty, what could be incorrect conclusions here. But you seem to be going to great lengths to avoid even mentioning story progression. What type are you using, if any at all, in this game? If it's meant to be NRaas SP, are you certain that SP in actually engaged under General Options and that EA's hasn't taken things over on you?

JohnL83, I do have an earlier save file. But I need to find out why this happened so it doesn't happen again. I finally got them all married again but it took all evening. I don't think its mod related because I haven't added anything new in a while.

igazor, I checked and I'm using SP version 267 and it is engaged. Its not EA , nothing has changed with my settings. I checked my married caste just to make sure that wasn't the problem and this is what its set at:

Match All Filters = True
Caste Filter: Sim Type = Married
Caste Age: Y, A
Apply to Household = (false)
Caste Automatic = True
Caste: Gender = (M, F)
Caste Inherited = (False)
Priority = 10
Species = Human
Pregnancy: Allow Can Be Pregnant = True
Pregnancy: Allow Participation = True

The only thing that particular caste should be doing is allowing married sims, both males and females, to be pregnant and to impregnate others. I'm pretty sure you don't want pregnant males, but you probably have that addressed with a higher priority Male caste. It doesn't allow or disallow sims from being married. Perhaps a different one does?

No that's the only caste I have that affects relationships. I dont have anything affecting Male pregnancies, only this caste, which I never realised before. So I dont understand why suddenly the game separated every married couple? I seem to get the strangest problems lol

You don't have Male and Female castes nor ones for Teens and other age groups? These are built-in, unless you have removed them. The Caste Filter option designates who is allowed to be in an automatic caste (e.g., Type Married), not what its members, once they are members, are allowed to do or are pushed to do.

On a few of the individual sims who were married but then became not, check on their SP > Sim Options. Anything showing up in (parentheses), meaning inherited from a higher level, under Romance and Pregnancy that seems unwanted?

Sorry igazor, I thought you meant castes I had created. I do have the standard, Male, Female, Teens etc.
I clicked on one sim that the game separated and under SP > Sim Options it shows:

Flirt: Affair Liaison (5)
Flirt: Affair Adultery Chance (0)
Flirt: Cheat Discovery (25)
Pregnancy: Allow can be pregnant (True)
Pregnancy: Allow Participation (True)
Pregnancy: Allow Participation Caste (1)
Romance: Allow Breakup (False)
Romance: Allow Marriage (True)
Romance: Allow Steady (True)

Then there are other options in parentheses, but they're with careers, skills and other unrelated options.
When I remarried them with MC, the Sim Options didn't change. They still showed the same settings in parentheses.

What I also don't understand is not only did the game separate them, but made them strangers.

Okay, those look fine to me. Only one other idea comes to mind. When you switched into the other houeshold, had the game only just loaded and had the game clock moved forward yet? If not, I mean if you loaded up a saved game and immediately switched households, perhaps the mods and their settings had not fully loaded yet so you really weren't running NRaas SP at all, but EA's and it decided it didn't "like" how you had things arranged.

I would try the household switch (and back) again on a copy of your now current game, except only once the game clock has moved forward and things are running as usual, to see if everyone stays partnered as they should. As you know, I use a Rotations caste to protect SP from doing things to "my" sims, a subset of those in my worlds, but no such protection for the rest of the town population. Even though I don't pay extremely strict attention to the other residents, I don't totally ignore them either and I've never had them all become unglued, so to speak, just by switching around decorating stuff. I don't think SP is the culprit here, I think SP just wasn't actually in control at that instant in sim time.

I hope this doesn't break igazor's theory, because once I saw, that some lovers-sims in my town have become for each other strangers, although until this their relationship was as positive as possible. It was an isolated incident that never happened again, but I was infuriated by the fact that it broke a very interesting storyline. At that time I didn't use any progressions, it seems that it was a error of the game. And something else strange was in the notebook where their history and reputation is written. It says they were having an affair, but it doesn't say how long or with whom.