Oh dear Lord. You did not force him to create a profile, scour the internet, and meet up with someone and have sex. That is not you. If anything you could say to him "i'm drinking because youre a cheater" But, thats not reasonable either.
Once you get sober and have some time away from the drink you will have a new confidence and crap like that wont sit well with you.
That said, what is your plan to stay stopped? Stay close to SR its a great resource to stay sober.