The only problem with rukia is that she doesnt seem to be even thinking about those sorts of things. If, and when, she starts thinking about a boyfriend then she would hotter. but as she is, she is just surpised and reactes bad when someone suggests stuff like that.
Imagine a guy hitting on rukia, only to meet renji. Renji fights him, beats him up, and then says hes ok. Then byakuya...