If it bothers you this much, know that you won't soon forget it. You need to deal with this emotion however you feel that you should. Communication is key here.
What's considered cheating and what's not doesn't mean jack, it's still going to hurt you to hear it regardless. In Office Space, when Peter thought the waitress (Jennifer Aniston) slept with Lumberg, although they didn't even know each other then, to him, it was still cheating. Things would've been okay had he never asked.
You can't forget that you did fight tooth-and-nail to get your boyfriend to admit this, this is also something to think about.