I'm getting my Sims to socialise with coupled Sims only one at a time so that the couple doesn't start flirting in mid conversation. But my current Sim's brother just visited her and arrived in a Flirty mood. I used Funny interactions to try and switch that to a Playful mood instead but he just got Very Flirty. During dinner with his son at the dinner table too it was just impossible to tone down his Flirty mood. I checked and there's nothing with the Romantic Decor effect around but regardless it was the same indoors and outdoors. He arrived on the doorstep like that. I deliberately didn't place cherry trees although I'd have liked to do so, because they are Romantic Decor now and that's just a no-no for me. I attach an image of one uncomfortable dining experience.
I experience this too since the latest patches. We are a household of four Sims and two of them are 'very flirty' right after log in and we just shot story pics in the session before (where they weren't flirty at all). There are just single flirty moodlets like the 'being/thinking about crush' stuff. The other moodlets are all regular ones.
And later that happened too, during normal gameplay with no romantic interactions.
