Well, in an unprecedented move, I'm going to disagree with Joyce Summers. Wow, huh?
I don't see Giles as a father figure to Faith. To the other kids, yes, but not to Faith. In the comics, he proposes more of a partnership between equals. Also, they have sort of bonded through the shared experience of having once been on the wrong side of this war. Plus, everyone knows I think Giles is hella hot. lol. I see no problem with a woman like Faith, who is in her mid-twenties by now, being attracted to a man like Giles, who is what? In his 50s somewhere? Cuz, really... he is quite attractive. They have some shared experiences and a shared purpose. I think it would be pretty natural for them to at least notice each other as attractive single people.