It's more the situation, since, um, Saber is about to kill Rin and all Rin can think about is how pretty Saber is.

... Also the fact that she's, um, very into Saber in a certain bit... and a couple other times from what I remember... and one of her lines is about realizing she likes girls, and, um... UBW Good End is basically her harem ending even though she's not the protagonist, um...

Edit: Um, yeah, eh-heh.