300 was more of a sausage-fest than BB/TDK, but...yeah.
That's the thing about Rachel in TDK. I think Bruce was in love with the Rachel he knew before he left, just as Rachel was in love with the Bruce she knew before he left. By the time TDK happens, Rachel was already pretty much off his bandwagon. The two just have completely different, irreconcilable ways of looking at the law.
You can see it in Alfred's face as he's trying to explain to her what Bruce and Harvey are doing. Rachel doesn't comprehend it at all, and Alfred knows that Bruce has already lost her.
Selina would definitely be a better foil for Bruce, because she breaks the law (albeit in a less righteous manner than Batman), but also possesses a moral code.