Of the two relationships I've been in, I've been cheated on twice.
First was in HS, my gf at the time was seeing my friend behind my back, because she lived some distance away and he had a car, I did not. I found out because she straight up called me asking where he was once, because he was late to a date with her.
Second was in the military, fiance breaks up with me after I find out I'm getting processed out of the Navy. No explanation as to why. She gets married to some Army schmuck about a month later. Kinda obvious she was seeing him while I was in Iraq.
I've all but sworn off women at this point, it's fucking ridiculous that people can't remain faithful, or pony up the balls to at least break off the relationship before fucking around behind their back.