I could have been the OW....
I was in a relationship with my exP from 16-22.
At 21 I met a man, D, through work. D had an exW, who he had (and has) an excellent relationship with, 2 DC (then 14 and 15), and a DP who he'd been with since shortly after separating from his DW 8 years previously.
After knowing each other as friends for about a year, I had major surgery. My own DP went on holiday the day I was discharged from hospital. D was there every single day. We both developed feelings for each other.
Nothing happened for 3 months. Then he left his DP and told mr he loved me. I had very strong feelings for him too. I left my DP a few weeks later.
We then didn't see each other - deliberately - for 2 months, so we could a. Be on our own and b. Figure out if there was genuine feelings, or just result of stressful situation evoking feelings.
After this time we started 'dating'. I then moved in @ 6 months after our respective relationships broke up. I met his DC after we'd been together properly for @ 9 months.
We've been together 8 years now and have our own DS. I have a great relationship with his DC (not as 'step-mum' at all given relatively small age gap) and great relationship with his ExW, their mum.
Feelings do develop for other people when you're in relationships. We both took our feelings as a sign that things weren't great in our current relationships.
But we didn't have an affair. Or lie. Or cheat.
As adults, surely you have enough self control to not have an affair but to separate, thing through feelings, then start relationship?