It can, and sadly does, run in families, too. My maternal grandmother abandoned my mother twice - once as a child, into my great-grandparents care, when she married my step-grandfather and once as an adult (although strictly this may not have been entirely her doing, as I suspect dementia played a part alongside her other daughter essentially abducting her in order to get her to change her will...).
Said other daughter walked out on her first family, on her youngest's 3rd birthday, when her oldest was barely 7 years old. For another man, with whom she went on to have two more children by. Her daughter has young children around the ages that she and her two brothers were when my aunt abandoned them for her "other man", and I was actually thinking how I hope she can break the cycle for their sake, the other day.
As others have said, though, why should father's abandoning their families and going on to have more children be seen as (sadly) "normal", though, and mother's doing so as "abnormal"? My grandmother and my aunt both abandoned their oldest child(ren) for the sake of men, with whom they went on to have other children. It screwed all of those children up - my mother, probably, the most because she didn't know who her father was, whilst my cousins' father raised them single-handedly for years (the oldest still refuses to acknowledge her as his mother, forty years later, whilst the younger two maintain a wary distance - she's not "Nana" to her daughter's children, for example, she's referred to by her name). My biological grandfather - had other children after my mother. My aunt's "other man" - walked out on her and their two children, for another woman whom he's started another family with. Are they the subject of whispers concerning "unnatural" or "abnormal" behaviours? Nope. But my aunt, certainly, has borne the brunt of the choice which she made (when her "other man" left her, after almost 20 years, her cousin said that it served her right...) and probably always will.
I don't understand how she or my grandmother could ever choose a man over their child(ren)... I know that as insane as my two have driven me over the years, I could never turn my back on them and walk away from them - but their father did precisely that. As have a lot of men in my circle of friends/extended family. Why? They're just as much a parent as their child's mother is, surely?