I think there are just certain Sims that don't ever find a particular interest in romance at all, and then there are those that apparently can't keep it in their pants. A great example of both of these is one of my earliest Sims on Sims 4, single mom and artist Loretta Lawrence. I had her spend most of her YA life caring for her son Daniel, and never once bothered having her flirt with anyone, except that one accidental embrace where it ended up applying the romance bar to a gay trans male Sim I created who never found any interest in here. Since then I've never seen her flirt autonomously with anyone, and appears to have no real romantic interest in anyone. Sometimes I've even seen her going out to eat alone at formal restaurants lol. It seems a bit sad but, it really gives her a personality that I always likened her to have; a single, independent mom who don't need no man!
Then on the other side, we have the premade Andre DaSilva, who Loretta keeps a room open in her house for him to live with. Not long after I started playing him, I had him start flirting with a man; the romance sparked, but it didn't go anywhere. Then he started flirting autonomously on his own with other people outside the household; men, women, young, old, anyone. He even dared flirt with my simself in front of my sim boyfriend which my simself casually rejected, when he was visiting their old apartment. Now he has both a girlfriend, and two side chicks that are Candy and Yuki Behr, and each has a son coming from him, and they're both in love with Andre and don't even know that they're both in love with him lol. He hasn't flirted with Loretta yet, but I imagine he would do it autonomously one of these days. Also, I'll be moving him in with the Behrs quite soon...and there will be hell. >:)