To give a bit of background my gf and I met first year of university. She was a friend of someone o was flirting with and broke the heart of when I asked my girlfriend out. In second year she had a panic attack because a girl I was living with who was absolutely gorgeous wanted to have me as a fuck buddy. I nearly cracked a few times but in the end I held off, that causes a bunch of drama that actually would have been avoided had I fucked her but that’s a different story for another time.
In the end, it was a friend from high school that I had kept in touch with that made me cheat. My friend (we’ll call her J) and I had been flirty when we had known each other in high school. That gradually over time went to being open about our relationships and sex lives. To the point we knew a lot about eachother. But there was always something building under that which we never addressed until my third year. She had gone to another university on the other side of the country but we had kept in touch and continued to be open. To the point that I was reading the sex blog she was posting at the time.