I'd just send a quick message along the lines of, "Hey yeah, no worries. I didn't realize you were shagging other dudes and that's not something I'm down with. Anyway, had a great couple of months with you, & all the best!"
She'll then protest / finagle / try to get you back, but at that point you can safely ignore. Generally it's good form to let someone know why you're stepping out if you've been seeing her a while. So you just do it in a way that highlights the reason, then ends it quickly and on an upbeat-yet-final note.
e.g., not ending in a whine, which most people will interpret as an opening to change your mind if they can just reassure you you're important enough to them. Instead you end on "had a great time with you. Good luck with everything!" note so she knows what the deal is.