The exact monologue after he stops kissing her because she's crying and saying "I cant" are...
"At the time I couldn't understand Bella. The attraction was there, but for her it felt so wrong. I remember feeling bad about kissing a married woman. I figured she felt the same since she was being unfaithful to her husband... which, of course, is a very big deal. But if I had known the real reason to her tears... to her persona... I wouldn't have kissed her that night. I would have called out for help."
So whatever this "real reason" happens to be is the crux of the matter. Can't say why what he did was so bad but his monologue seems to imply it wasn't the infidelity that was the issue but something bigger.