Just sharing a thought here: you've been together for 6 months. I don't think you're quite at the stage where the attraction has morphed from purely physical into something deeper. As you continue in a successful relationship you'll generally find that attraction grows stronger because it becomes based on more than just physical appearances. It might be a warning sign that you haven't gotten there yet at this point, too.

Also - given the fact that you refer to him as your "boyfriend" but say that it's still "casual" plus the contents of your last paragraph (a seeming mismatch in personality) seems to indicate that you were already not really "feeling it" and this is just a case of your brain rationalizing a way out.