Lots of insightful posts on this thread which makes a change given the topic.
I've had an affair (both married) and been an affair partner (me separated, him married). They were very different and a I think every relationship, whether an affair or not, is unique given that the individuals involved are unique
My affair was of the wake up call variety. I was unhappy and had been for a long time but fear, obligation & guilt kept me in my marriage, I hadn't even considered that ending it was an option. The affair was just sex, he was much younger than me and though I was hit hard with limerance I didn't even particularly like him. It lasted 6 weeks and I knew that it would happen again i.e. I would meet someone I was attracted to and want to act on it. I decided I needed to leave my marriage but it still took me two more years to do it.
The week before I told my then husband that I wanted a divorce a married ex-colleague who I was out for what I thought was a platonic lunch with told me h'd like to be more than friends. He didn't know I was about to leave my husband. He'd had a couple of purely sexual affairs that had probably enabled him to carry on in his marriage. He didn't think he was unhappy enough to leave and also suffered from FOG.
Long story short we fell in love and after a year he decided he was going to leave. It took another year for him to leave during which we were mainly low contact. Lots of heartache all round. I don't feel any guilt about how our relationship started and am open with people about it. They had no children, maybe I would have felt differently if they had, I like to think I would've never got involved with him if they had. But, I know he feels guilt as he knows he should've been stronger and ended his marriage sooner, the first time he found himself looking outside his marriage and that he put me through a lot of pain. I tell him though that that was my choice, I could've walked away at any time (in fact I did a few times and dated other guys).
Interestingly, I really don't think he would cheat on me nor I on him. That might not sound very rational but we're both very different people now and our relationship is very different to both our marriages.
Neither of us have been financially ruined, my kids are fine, my ex is fine, his ex is fine (and is better off without him if he can treat her like he did).