I'm so late to this but have to leave this response because i always see comments about cheating on reddit and people demonize it and want to make it black and white. Heres my POV from doing it:

Sex is the most powerful human instinct. Reproducing our DNA is what keeps us alive as a species. Go ahead and say we've evolved past being animals, but when it comes down to it biologically there's a reason the desire is so strong and tempting -- we need it to be for our survival. Not to say we're not smarter than animals, and can exercise willpower, but that's always lurking in the background.

I cheated because things were kind of rocky in the relationship with my GF and I started to wonder if I was settling and if I was still even attractive to everyone else after being in the relationship a long time. I'd also never had amazing luck with girls up until that relationship (I've learned a lot about myself and that's better now). Should have just broken up, but it's so hard to do that when you're on the fence about a relationship.

There was alcohol involved. She was gorgeous, and was moving to another city. It was a perfect situation, a one time thing that happened in the moment and that I had no intention of repeating. Sex does not always = love. Sometimes it's just recreation.

I immediately felt awful about it and decided not to tell my GF and just to try to figure things out with our relationship. Some people (she included) said I should have told her. I get that, but to this day I still disagree. It's bad enough to do that to someone, and to me the punishment was that I had to push it deep down and live with it and never tell a soul. Telling her seemed like an unnecessary infliction of pain. I loved her; why should she suffer a long stretch of pain and doubt for my moment of stupidity?

When she found out it was the worst thing I've ever experienced. I felt like a terrible person and that guilt will always be a part of me. That's what karma is: just guilt and repercussions, nothing magical.

I would never do it again in a committed monogamous relationship. Hurting someone like that is not something I can stomach. However, I'm still debating if I want to be in a monogamous relationship again.