Lixisa17 Neighborhood Stories does not autonomously marry sims. Although, it does have move in and move out, it doesn't move sims in and out of existing households nor does it combine households. It moves the entire household in or out and generally does not affect sims that are already on a lot.
What you had happen is only possible with a mod and the mod is usually MCCC when you have the MCCC options that make up its story progression on. So, if you want to stop that behavior, you would be best served by going to the MCCC Discord to ask how to turn its story progression off if that is what you want to do. You can find a link to get there by going back to wherever you downloaded it (Patreon or Deaderpool website) and following the link there to get to that Discord.
Other mods may do similar things but MCCC is the only one that I know of that fits what you describe.
Hope this helps.
