Interesting thread. My mother had several affairs when I was a child. I guess if she had posted on mumsnet about her marriage to my dad she would have been told to leave him. He's not a bad man, but they were a classic case of marry at haste repent at leisure. At the time I didn't know that my mother was seeing other people, and I don't think my dad did. But it was still a confusing environment to exist in.
The person having an affair, IME, is often emotionally unavailable. The priority, if you will, is the relationship occuring outside of the home. When the affair is going well then of course everyone at home benefits, when it goes badly/ends everyone at home suffers - but without the benefit of knowing why the rollercoaster is happening.
There were no rows at home, but a constant undercurrent of tension. I knew that my mum was lying to my dad about things and that used to upset me.
I could never do that to my kids. To cheat on my DP would be disrespectful to them/our family unit. It is a million % times better for kids to see their parents have an honest split than for them to 'exist' in a tissue of lies.
I know that no one is perfect, but infidelity (where kids are involved) is a shitty thing to do however you look at it.
My parents are divorced now, and are both very happy with other partners. I just wish that they'd taken the step to divorce about 15 years earlier than they actually did.