In Pitchfork (TS2) I have a romance Sim (f, a) who never had much luck in finding the right one - two bolts at best. I finally settled her with a very respectable gentleman, an Education Minister; they got married and had a baby. Then one evening she came to the Academy and met a townie; they started talking, then flirting and, before long, she jumped into his arms. I was playing another Sim at the time so I could only watch them; I did not try to interfere either, of course. I checked her out later and she had three bolts and a heart for the townie.

This could happen in TS as well; I don't remember it happening in TS3 but, at least, I played a lot of that game. I am still inexperienced in TS4; I saw Sims flirting a bit in various places but never doing anything more, even the steady/married couples (who don't seem to go out much together anyway). Is it even possible?

Sims in TS4 are not deep. As you play, you will see adult sims flirting with toddlers, family members, ect... It's a bug that has been around since TS4 was released. Maxis has made 'attempts' to fix it, but it's one of those bugs that never goes away.

There has been a lot of chatter over the years about the incest bug. Maxis is "yeah, whatever..." about it. If you don't want this kind of crap going on in your game, I don't know if there is a mod for it or not. Probably?

I have a 100% cure for issues with toddlers.... babies go straight to child in my world. No toddler BS for me thanks.

I also have a "plethora" of mods, MCCC tuning and lot traits to combat random flirting and the Lovestruck jealousy traits seem to help..... I think

Not a mod for it, but a method.

The root of the problem is in the Playful Mood XML. As all other moods go up and down from low, to "very" high - to sometimes even killing sims. Playfulness in adults is allowed to, and often does, transition to Flirty, coded so in that very same Playful Mood XML. And now you see the makings of a calamity when Playful kids interact with Playful adults, those spaghetti coding wires soon get crossed, and it's time to call in Child Protection Services while getting over the shock of the child's new romance bar for their otherwise innocent parent.

So the method to end the madness, find a way to keep those kids' moods high as a kite on anything other than Playful. Should then never experience the shock again. I have a mod to keep them maxed out on Inspired. Does the job nicely, and the family makes a decent profit too selling all those kids Inspired masterpiece doodles. Double-whammy upside. Joy.