I've had two relationships that lasted longer than a year and a half.
Not once did I consider cheating as an option, and between the two it took like two months before I was ready to give somebody my heart again.
I don't think he's considering cheating, only that he's found a new love interest and is conflicted as to whether or not she would be a better fit for him.
In my opinion, there's nothing wrong with the way you feel. You aren't a horrible person because you are attracted to someone other than your girlfriend (you can't exactly control those emotions all the time). What will make you a horrible person is how you choose to go about handling it. As long as you don't cheat on your girlfriend and just break up with her, then I don't see it as a problem, personally.