I must say I have been very frustrated with 'autonomous attractions' and behaviors, I think since the patch before Love Struck (?)? I seem to be having more romance bar issues lately, too. The sims in my household have all of their settings set to the opposite sex. I have had issues with romance bars appearing randomly, sometimes with the same sex, and even at times with family members. It disturbs me to no end. I have just thrown my hands up and started just having them ask to 'just be friends' when this happens, but the empty romance bar remains. I finally resorted recently to adding the UI Cheats Mod to remove the romance bars I didn't want, then I removed the mod.
But... just a few days ago I went in with all of them and changed the very last 'attraction option' - in the last tab to the right, the very bottom option that is set to 'yes' by default (something about whether their preferences could be influenced by talking to others)... so I set it to 'no' for everyone in the household. I don't believe I have had any of the random romance bars appear since doing that, so far, so we'll see. You can try it if you want.
Either way, I am very frustrated, and sometimes disturbed, by this. I do not want to enter my household and find that someone has a romance bar with their stepfather, mother, or even married sims (sometimes within the family) when I have put their settings with both to not be attracted to 'taken' sims. Not cool.
As an aside, adding that mod to adjust is the extent of my Mod/CC use, and I remove it after, take out the localthumbcache.package file and do a game repair. I truly don't think it's any fault of my game or save. I'm on Mac and have all packs.