About 7 years into our relationship she was very drunk and with a friend of ours (also female, also drunk) and made a pass at her. I had been aware for a long time that she was bisexual but we were in a monogamous relationship so thats that as far as I'm concerned. She made a drunken pass at our friend, who promptly bolted and text me to talk to my SO. I was at work but knowing my other half so well, I had suspected she had started to have feelings for our friend and knew what the text meant.
I was gutted, we had plans for family, we weren't married but our plans were laid out.
I did have some sympathy for the fact she had never got to explore the other side to her sexuality because we started going out a 18, the bit that hurt the most was that there were some genuine feelings behind her actions.
I seriously complemented leaving her, but after 6 years together it was impossible to cut ties without properly talking it out. After a day or so I went home to talk. I had never seen a look of shame on someones face like that before (she was honestly more sorry for hurting me that the potential of losing me), she didn't try to talk her way out of it, was honest about what shed done and was genuinely sorry.
It took us the guts of a year to be back on track, at times we had difficulties and we rowed a fair bit which was never like us BUT we talked a lot more and forced ourselves to be more honest in general than we had been previously.
We are now together 15 years, have 2 kids and one on the way and our good friend is thankfully still a big part of our life. I completely trust my SO that it will never happen again.
For the most part I think that when someone cheats or tries to cheat they will probably be a serial cheater. If you have doubts (beyond just the self worth/confidence hit you take after something like this)... just leave, I wouldn't have had children with her had I ANY ongoing doubts.
I suppose what worked for us was hard work. we had to be very honest and open, im nowhere close to perfect myself and can be a pain in the arse to live with at times. The discussions were not just a one sided lamblasting of my SO about faithfullness, it was about our entire relationship and the way we both conduct ourselves towards each other.