Op, you are getting a lot of bad advice here from women who have no idea how men think.
Roughly speaking, 15% of men are dogs, will never be faithful. 15% will never cheat, even if their wives deny them sex, affection. 70% are in the middle, they are human, they can and do make a mistake or give into temptation. My guess is your DH falls in the middle, but what do I know. How's your relationship now, does he treat you well?
Point being, you can dump him but you are statistically likely to end up with someone who can also cheat on you.
Finally, I would recommend confronting him. Men compartmentalize sex, many married men would cheat if they could get away with it, and if he thinks he has he is more likely to do it again. It's just sex to men, it doesn't feel like a betrayal unless they see and feel the consequences. It doesn't have to be dramatic, but simply "I know you cheated on me with ______ and if you cheat on me again, I am leaving" It will scare him straight and make him realize what he has to lose.