@crinrict I've only had them together in a couple of lots since I got them into couples (their home lot and one other lot) and it happened in both. I have not had this happen with NPCs, only with the other members of the household, but I haven't had them interact much with NPCs. I have a Maker/Freegan/Creative, Art Lover/Gloomy/Creative, Green Fiend/Recycle Disciple/Loves Outdoors, Self Assured/Good/Outgoing, High Maintenance/Vegetarian/Lactose Intolerant, and Ambitious/Cheerful/Bro. I left them all with the default orientation, and haven't had any of them interact with anyone romantically before their current partners. 5 of them are all nearly 100% in friendship with each other, while the 6th I moved in a little later and is high friendship with a couple of them and just acquaintances with the others. None of them had any romantic bars until recently when I coupled them up, and I've only told them to do anything romantic when they are one-on-one with their designated partner. Before they were coupled up, if they got flirty from watching Romance or something, I'm not sure if they were flirting autonomously with each other or not (as there wasn't anyone to get mad about it), but they never developed any Romance bars if they did. They live in a small house with one living room, so they are often all together and will autonomously get flirty interacting with their SO or from watching something on TV and then go and flirt with one of the the others, making both of their partners mad and dropping their romance bars if they are in the room. It is a nearly constant occurrence.
I haven't had a chance to test this out with another save or anything yet, I'll add that info in when I do.
