My Sim dated an Unflirty Sim for a while, and I also had that Hold Your Horses moodlet when he tried to hug her. I thought it was my Sim sneaking in a romantic interaction when I wasn't looking, as he often does, causing that moodlet, but apparently he really contained himself this time... huh. Anyway, there was this weird thing where she'd often show up in a Flirty mood at his house - no idea why, because her traits are Unflirty, Loner and Dog Lover and my Sim has no romantic decor whatsoever. But that made it very easy to nonetheless pursue a relationship with her. From my experience, having a romantic relationship with a non-played Unflirty Sims is hardly noticeable and not an issue at all. But I have never actually played Unflirty Sims, so it might be very different in such a case.