I guess my mom had an accidental emotional affair.

My dad wasn't there a lot of the time, so my mom basically raised my sister and I with little to no help. So my sister and I were basically her entire life since she was a stay at home mom with an absent husband.

My sister became best friends with another little girl in kindergarten. My mom would always take her to play dates with this girl. This girls dad was a cool guy who was sort of in the same position as my mom, he had an absent wife and was a stay at home dad. They bonded over time and became best friends due to their very similar circumstances and ended up falling for each other. Never physically cheated on their spouses, they both felt that they needed to end their marriages before they made it physical but it was all emotional.

They're still together and he is now my step dad. Took a long time to forgive them both, my sister blamed herself for a lot of it but we have all come to terms with it. I guess this is the closest you can get to "accidental cheating"