It takes work (so much work) and if both people aren't committed to it then everything unravels very quickly. I'd be curious to know how long they were together. I, for one, would never want to be in the dating pool again. I would probably stay a perpetual bachelor.
They have been together for for about 15 years, which isn't as long as you would expect but you would think the itch might have been settled by then before they decided to have kids. I guess it might just be a late variant of the mid life crisis!