To answer directly your question, not I don't think cheating is OK, but neither do I think it is fair that on marriage ending many men lose their children.
My view my be blinkered by my own experiences, but I have seen over and over and over again, children dragged into the issues between their parents, and to a degree, the older the child, the more embroiled they become, as they are spousified by the RP.
Men do actually love their children, and I genuinely believe many men do not want to leave them behind.
Personally speaking, having seen what happened to my own husband, I can see that for a decent man, then end of a marriage can mean the end of life as you know it, no children, no home, no foreseeable way of getting one big enough to home your children and yourself.
And his story is sadly far from unique, in our case, she had an affair for 2 years (emotional/physical no-one knows), he met me (sort of as we had known each other for a long time), there were other factors that came into play, that made him see his children were being damaged by him staying (he should have seen that long before he did but there was an event not involving me that catalysed it all).
He moved out, into a 1 bedroomed flat (heavily subsidised by a friend as he was effectively homeless) - then we started a "relationship", then after time he moved in with me. I was the catalyst for him leaving, but not the cause of the breakdown of his marriage.
But I don't think her affair was the cause of the marital breakdown either, their marriage was over long before that, they weren't actually speaking when they got married, it was an attempt to "fix" an already broken relationship.
Its very, very sad, and they would have been better off parting long before they did. The children have been held hostage to the hostility their mother holds for their father and are badly damaged, this goes on into adulthood.
I don't know what the answer is, but I am sure of one thing, DHs ex had no respect for him, and by the time he left, he was so immune to everything, he couldnt care less what she did, although that cant have always been the case as he found out about her affair by following her one night.
And in this toxic environment, were 3 children.