When I was sixteen, I cheated on my first boyfriend by kissing his best friend when my ex was on vacation in Mexico. Doing so caused me to realize how much I cared for my boyfriend who until then I treated like shit. (He was a hard core pursuer who won me over despite all my efforts to push him away)
So then shit hit the fan, and he door slammed me. Needless to say this fucked me up in relationships for a long time. I haven't cheated on anyone since. If I hook up with someone else, it is because I have zero intention of revisiting the previous relationship. I also let people know before and that its done.