I kissed a boy while on a break with my boyfriend of the time. Apparently we had different ideas about what that meant, which is the main objection I have to breaks: it's like breaking a toy so that other kids can't play with it. Personally I don't qualify this moment as cheating, but since my boyfriend at the time did I guess it counts.
I was on holiday with my boyfriend at the time, who I will call Chris since that's his name. On the second day of the holiday, Chris came in blind drunk at 1am, woke me up, dumped me, screamed at me for two hours so loudly that he woke up the people sleeping next door, and tried to take advantage of me sexually while I was still crying. He also threatened to kick me out of the apartment, meaning that I would potentially have ended up homeless for two weeks with only a rudimentary knowledge of the language. Like an idiot I took him back after he decided he still loved me when he felt jealous that another boy started hitting on me.
So, we're back together and go out with this friend, who introduces us to another guy(let's call him Steve). Steve begins hitting on me, which I politely try to ignore. Steve continues and I become increasingly uncomfortable. I ask Chris to come and sit with me hoping that Steve would realise I had a boyfriend and leave me alone. Chris however refused and just sat there smiling at Steve while I tried to keep Steve's advances at bay. (Measurements might be relevent here: I'm about 5'5 and a little chubby but very weak, Chris was about 5'8 and lanky as a streak of piss, Steve was about 6'2, built like a brick house and probably could have crushed me if it got that far). Chris goes outside for a cigarette and I follow on the pretence of getting fresh air - really I want to know why the hell he isn't helping me fight this guy off when I've specifically made it clear I don't want to be hit on (by this point I had already pushed Steve away twice). At which point Chris says:
'We're on a break, so leave me alone.'
It felt like such a slap in the face. Hadn't he told me just that morning he still cared about me? What had I done wrong? I walked back in feeling numb and Steve tried to kiss me again when I sat down. So I let him. I just thought, 'Sod it, what's the point?' I went out, had a lovely time and got back to the apartment at 3am feeling euphoric - until I got in the apartment and discovered that Chris had seen me walk off with Steve and the other guy (both people he'd been out drinking with basically every night) and lost his shit.
So that's my story. Some people would count that as cheating. I don't. The reason I did it is because over the course of the holiday (and on a grander scale the relationship - the more I look back on that relationship the more questionable stuff I remember) I was made to feel like shit. Most people don't cheat for the giggles.
TL;DR: He made me feel single, so I acted single.