Ideas
I also have a terrible problem with most of my sim households with this issue. I've been struggling with it since installing LS last May. Thinking I've got a learning curve for how the system works, I had not upvoted this thread until today.
The worst issues with keeping couples happy are with sims in existing saves that I had not set turn ons and turn offs for. I'm experimenting to see if adding them helps but the biggest issue for me is that I do play rotationally and no matter what I do, any headway made is lost if I rotate away and return a week or so later. I reloaded my large ranch lot to use that household for the new event and both husband and wife had completely empty romance bars. They weren't that way when I left the household. By the next morning one of them had developed a fear of being cheated on. Something should be done so the relationship in played households doesn't deteriorate for rotational players. The constant uphill battle when reloading a lot after rotating back to it in a save gets old really quickly.
I play on PC and have never used mods/cc.