Anyone who's been in more than a couple of relationships generally has their cheating story to tell.
When I was 18 I was dating a 16 year old who cheated on me twice with a guy 10 years her senior. Since it was my first proper relationship I was stupid and took her back twice, until one night when I got drunk and hooked up with 3 different girls in a club and rang her and told her I never wanted to speak to her again, and haven't since