I have had the same issue (for years literally, but it has gotten worse with all the romance/ sexuality settings). Added a married couple with their two sets of parents, and one grandparent, to the same household. So three generations. Bad Idea.
To begin with the grandmother was obsessed with flirting with her daughter-in-law, so her own son was angry with his wife constantly. As as soon as I manage to build the relationship again, there she was instigating again.
Even worse is that it was ALWAYS the grandmother but the son always got angry at his wife, with moodlets suggesting she cheated, even though she did nothing wrong. It's his own **bleep** mother he should get upset with.
Anyway, the grandmother's son, who was so upset at his wife when his mother is flirting with her, then got wishes to flirt with his own sons wife... Thankfully he never did automatically.
Since the grandmother was flirting with someone of the same gender (despite having a boyfriend), I changed the settings for her to only like men. That did not help. I could literally have her selected and she would automatically talk to sims, to then randomly add in a flirt/ romantic interaction directed at the daughter-in-law, with her having only interest in male sims in the settings.
To note is that I would always "cool off" the relationship that was build by this, to be "only friends" again. I would assume there should be a behind the scenes cool down where flirting between the sims that have agreed to not flirt, would be prevented (at least without my input), but I have never found this to be the case.
Even in instances where I play a single sim that cool off a romance, they will then get the crush phone call to ask if they can be more than friends and/or get flirted with again by the same sim who just agreed to just be friends.
Anyway, back to it. The final straw was when the father to the young married couple decided to flirt with his son-in-law, randomly in a conversation with him AND his daughter, TWICE. He had a full green bar with his daughter by the way. I don't understand how that is a thing. Like, you love your daughter, and yet you decide to randomly flirt with her husband in a conversation you're all in? Does not seem so loving to me.
I could maybe understand it if perhaps the father and daughter had a not so great relationship, or if they had angry sentiments between themselves, or even if the father was gay/ bi, (but he was set as straight to prevent this after the grandmother debacle).
Also, because the sims in this instance are very old/ played for a long time, they have high skills. So charisma is high, and since they all like each other, romantic flirts never fails. Even if their own partner is in the conversation (which I do find strange/ annoying). The partner get angry for getting cheated on, but the one getting flirted with does not reject (even though I still think the one getting angry should not always get angry at their partner if the one instigating the flirting is not them, especially when the moodlet on aways suggest the partner cheated/ made a choice to cheat - not just get randomly flirted with).
So the sim getting flirted with never rejects, but I don't think that matters either, because even when a husband or wife rejects a flirt, the other part in the marriage always get angry with their own partner regardless in my saves.
After all the chaos and no-one being able to be happily married anymore (except the grandmother because her boyfriend did not live with them), I decided to finally split the household. Which had been my intention from the start, I just wanted to up the relationships before doing so as I had added them from my Library...
Now don't get me wrong, TS4 is so safe and sweet it lack consequences and drama if you will, something I sorely miss from the game, but this is not the kind I'm missing. Especially when it happens is all of my generational households. Does not matter traits or relationship with kids etc. Even settings for sims don't matter, they will flirt with the gender you set them not to regardless. I guess maybe setting the romance off completely might prevent it? But like in this case the worst offender (grandma) had a boyfriend so if I set romance options off on her she would not be able to be romantic with her actual partner either.
Funnily these couples have been my "forever" couples if you will. Since 2015, and their older equivalents (in other saves) have always had the best relationships (in the marriages), so for once I actually wanted the loyal pop-up from GrT on them, but since they where constantly being flirted with by in-laws, it was impossible to get on them.
