Ugh cheating. It's a red line for me. Kudos to those who try to work through it / discuss it but I never would. In my mind there is simply no excuse which justifies it.
When I was sixteen I found myself in A Very Serious Relationship with a guy a few years older than me. His seemingly all consuming love for me bordered on disturbing but I was very susceptible to the attention having been ignored by my parents for most of my life.
Six months in he slept with a girl he met on holiday when he was drunk. Filled with remorse, he came home, confessed and tried his best to rebuild things with me. I tried so hard to forgive him and move on so loathe was I to walk away from this great and true love.
It was just physical.
I can't even remember it.
She was so beautiful, it was only lust.
She was plain, I was drunk or I would not have looked at her.
Etc.
None of it made the pain any less awful. He had been intimate with another girl.
(Some time later he cheated on me again and i dumped him this time for good.)
As life lessons go, it was an early one, it was a harsh but effective one. I never ever tolerated betrayal again. I decided I just didn't have the coping skills / ability to forgive that sort of betrayal and I was never going to try again. Or I simply had no desire to try.
Someone who loves you doesn't hurt you. The reasons don't matter. The excuses don't excuse.