Little relationship problem here..

My Sim is currently in a relationship with J Huntington III whom has the non-committal trait and I've played other NC sims in the past (for some reason the're usually my favorite lol). I've never noticed them being extra flirty with other sims or had any real problems even with sims like Don Lothario who also have the flirty trait and the serial romantic aspiration. Mostly they just get tense which can indeed get annoying but mostly because it can interfere with getting them in the right moods for work this can be avoided if you fullfill wishes and purchase the Carefree reward trait for them then they won't be affected by the tense moodlet. Another way to clear up the tense moodlet is to have them change jobs frequently.

Honestly the most annoying thing apart from the tense moodlet is that they will autonomously discuss their fear of commitment which can put both sims in a bad mood lol. J always likes to do that at the worst possible moments like right before I'm going to have them get engaged or right after she told him she was pregnant lol.