I guess I'm old-fashioned, but I think a relationship can be serious without sleeping with someone. If they hadn't thrown in that line about "someone would know if I were out at night," I might have felt better about it. But, the fact that she is sleeping with him makes her seem... two-faced, for lack of a better term... for telling Bruce that she would wait for him, and be there if he stops being Batman. If she doesn't want to wait and loves Dent, fine, just don't deceive Bruce about it. I think you could portray it as a long-term, serious relationship without coming out and telling everyone they are sleeping together... in fact, the idea that she might NOT be sleeping with him and doesn't WANT to accept his proposal right away would provide more emotional depth and credibility to her supposed feelings for Bruce.
As it is, friends of mine - of all different moral backgrounds - just come out of the movie hating Rachel... as a spineless sycophant.