I wonder if they nerfed this. I have been playing a 10-12 household rotational game since June without mods, and have not had any sims develop romantic connections while I wasn't playing them. They do develop lots of new friendships due to walking around in the background of lots where I'm playing other sims.
I even have Salim Benali in the Partihaus club, where it turned out one of the club activities is "Be romantic with everyone," and they show up anytime I visit a nightclub. I keep thinking I want to take him out of that club, but he never does seem to actually be romantic with anyone. Even with that club he hasn't yet developed any pink bar with anyone other than his wife. I do notice that he often brings his wife (not a club member) to the nightclub with him. Perhaps that's just the way it works best for them B)