The most unattractive sims are the ones who ignore me for their phone or my own computer... Oh, wait...
I don't like playing romantically, so when I decide for whatever reason that my currently played sim needs a partner, I don't want it to take any more time than necessary. That's why I tend to avoid traits that make dating even more of a chore than it already is, such as Loner, Unflirty, Mean, Noncommittal, and Hot-Headed. I once tried to date a sim who would constantly reject friendly and romantic interactions and seem uncomfortable despite high relationship bars - turned out he was a loner, so I dumped him. Yeah, let's not do that again any time soon.
My own simself is a tough nut to crack, being both unflirty and a loner. If I ever feel like hooking myself up with someone, that's going to be a challenge, and the only time I'll ever be open to it.
