It Was Just a Tiny Slip!
Is it irrational for someone to not speak to you for three years? It depends on why they’re giving you the silent treatment. In this person’s case, they weren’t speaking to their ex-fiancée because she cheated on him. That sounds like a very good, very rational reason to not speak to someone, though we wonder why they didn’t block their ex.
What really tickles our funny bone is how the ex wants the OP to be the bigger person. As if. If we were this guy, we’d go the super-duper petty route and probably egg her, her lover, her car, her office, her boss, and everyone who is and has ever been slightly affiliated with her. Minus ourselves, of course.