This happens to me all the time, in any save, with any YA to elder, though sometimes also with teens (teens only flirt with other teens though). Whether or not they've got the loyal trait (which is probably still broken), any sim in a relationship will autonomously flirt with any random other sim regardless of romantic or woohoo preferences, causing the hurt moodlets if their partner is in sight. It's absolutely annoying and inappropriate that this just happens with autonomy off, and we have to babysit our sims to try to prevent spontaneous cheating for no good reason. The sims don't even have to be in a flirty mood for it to happen, at least for me.
I'm adding images of a Mother becoming flirty randomly, in conversation with her child. I mean, that is so disturbing. I'm also highlighting the moodlets explaining why she's so flirty and one of those has a teddy bear and says it's boosting her flirty emotion. It's horrible and it needs to be addressed urgently. It's worse since adding Growing Together. In this case the mother was nowhere near her husband and hadn't spoken to him that day. I deliberately don't let the parents speak now while the child is present because the child gets pulled into the flirty conversation. She made breakfast and suddenly became flirty. She sat down with her daughter and her flirty emotion got worse. She lost relationship with her daughter as a result even though I kept control of the conversation and chose funny interactions or chatting about school. I ended the conversation and her daughter headed to school. Her flirty emotion kept increasing and the image shows everything was setting it off from the fridge to the pool.
I have the same problem. They flirt excessively, even when they are not in a flirty mood, I asked and I wish the game give us an option to stop the romantic interactions, something that you can turn on or off. Believe me, I'm not a baby or a nun, but repeating and repeating the same flirty things all the time, or most of the time's annoying and bothers a lot. Frustrating. It pissed me off so much that I had to divorce, delete and now I don't play with couples. They talk in a flirty way to their children, conversations become awkward and/or offensive if a third person joins them, even if they are part of the family. Btw, I don't go anymore to the Romance Festival. I think it has happened after the two last updates. Before only happened for the ones who had "romantic" traits. If you can call that romantic, lol. I don't choose romantic traits anymore, all the contrary, I tried unflirty traits, no romance or genre interest, no woo-hoo, etc, and still they flirt randomly anyway.
