I don't regret mine, but it's a little different to other stories shared so far.
I didn't have children and I was the one married. The guy I had the affair with had a partner (which he didn't admit to) and a child which I knew about but assumed lived with his 'ex'.
He was incredibly supportive which is something I needed at the time to get me through a stressful work situation and a lot of exams to gain promotion. My exh was jealous I had a better career and tried to drag me down constantly. Our relationship got worse and worse and the other guy tried many times to persuade me to leave.
It wasn't a massively sexual relationship, we didn't see each other outside of work as we didn't live near each other and neither of us drove, and it went on for about 3-4 years on and off. He termed it as feeling like an obsession, like he couldn't be without me in his life. I felt a calm that I've never felt before when I was with him, like he was my soul mate. The way he made me feel made me realise that I hadn't enjoyed being with my exh for years and I was just staying because it was easier.
No one found out about the affair so there was no big drama, no one with hurt feelings etc, no scarred children. I caught my exh several times during my affair trying to cheat on me which he denied (sad, desperate messages on dodgy websites begging strangers to be his fuck buddy, photos of his dick on bisexual websites, wanking online with strangers etc). Eventually, after a huge argument over nothing, he attacked me - I left that night but don't think I'd have had the strength had I not had support from the other bloke.
It was the beginning of the end for my affair too, he was badly affected by the violence against me as he'd always suspected domestic abuse and I'd laughed it off. He thought I had lied and been in a constantly violent relationship but it had been a rarity up to that point. He had issues at work which led to mental health problems and eventually to him walking out of work one day and never coming back. We eventually lost contact and I still miss him, but more as a friend.
It seems to me that we were just a bunch of very sad people in bad places in life, definitely with the wrong people. My exh was in a serious relationship within weeks of the divorce being filed. I had a few shitty relationships which made my marriage seem better in hindsight. Then about 2.5yrs after my divorce I met my DP. He's wonderful and a very caring, understanding person. He knows about the affair I had and trusts me to never cheat again (I definitely wouldn't). We have a baby due soon which is something I would never have considered having with my exh as I want my child to see a settled, loving home and understand what you should have in a relationship. I saw my mum settle for less in shitty relationships and it screwed up my judgement and what I was willing to accept.
Although I don't regret it, I shouldn't have done it. I should have had the balls to end my marriage years before it all came to a head. But there's no denying my exh and I are better off without each other.