For all I know the difficulty and bitterness of divorce could make it harder for separated couples to co-parent, leading to more single parenting in families that start as married.

I'm not sure about that, I'm open to persuasion. But I'm not persuade by people saying {it's obvious}.

Actually, I should retract my previous statement. I don't really know what I wrote that, maybe something I'm personally reactive to.

As far as I'm aware what the family relationship counsellors are saying these days (the ones I've spoken to anyway) is that it is conflict that harms children.

It matters less whether the parents are together. What's important is that children aren't exposed conflict.