Say something like this: You knew at the beginning how this relationship was going to work, we hang out and have sex, and that is all it is.
I am ready to move on, and since this relationship was built upon the mistrust of my infidelity, we have no future. You should have known that, as it can't be anything else. I guess that I will see you around, but this is how it is and how it must be. Good day.