Katie, the sex is boring because you and he have let it get boring. Cheating is a very extreme step to take. Try some other things first, like new positions, role-playing, watching/reading porn together, maybe some light bondage. Cheating is lousy choice, because somebody will eventually get hurt. If you love him, don't cheat on him. If you really want to cheat, then you don't really love him. End the relationship and move on.
Good decisions come from experience. Experience comes from bad decisions.