Same here. Doing the 100 Baby Challenge, so this kind of situations happen often, so I made sure to gradually complete the Serial Romance aspiration. She has the Player trait.
She just had a First Kiss with a guy in a bar, and one of her previous romances was there, and got Furious about Cheating sentiment. He is one of the boyfriends that were needed for the aspiration, though, and he did not seem to react to the flirting that happened before the kiss. Nor to the flirting after the First Kiss. She asked the new guy to go woohoo with her in a closet, and the old boyfriend didn't react. He also doesn't seem to get angry, he's in a happy mood.
She finished with the woohoo, came back downstairs, did Smooth Apology on the ex, and the Furious about Cheating sentiment vanished.
I think the Player trait is working for her, but Sentiment system wasn't coded to take it into account and needs looking into.
I have all the packs, the game save is new (Jan 2021), I use SimsInfo and NoMosaicBlur mods.
