As an unflirty person in real life, I often have trouble telling if someone is flirting with me or not, and I've been told a few times that I sometimes flirt with people without even realising it (or rather, I unwittingly do stuff that others might misread as flirting). This has made me self-conscious in that regard, because obviously I don't want people to think I'm flirting when I'm really not, and so I also avoid any kind of physical contact that might be misinterpreted. Maybe that's what the developers had in mind when they made this trait? It makes sense to me at least, and I do appreciate that my simself reflects the real me so well.
As a side note, I'm actually trying to hook my simself up with someone at the moment, and since I'm not only unflirty, but also a loner, I have no idea if that's even possible. :D But I'm looking forward to the challenge!