Some people will stay with anyone they feel they're "in love" with regardless of surrounding circumstances.
Most men for instance do not get married to have a family. They marry the woman they love and (accepts) what comes with her. This also explains why (some) men abandon their own children if the marriage or relationship fails because they do not want anything to do with their ex-wives.
Not many men who have children would require that their new woman love their children. Maintaining the peace is their top priority. As long as everyone is "civil" when they're together the typical man would be content. Women want men to be more emotionally invested in their children.
Recently a man told his wife that she came first along with their marriage. Her response was "my list is kids, husband, family and then friends". Some women would say their order is kids, family, friends, and husband.
This explains why so many men see having children as being demotion on the priority list. While the men see children as being an (extension or by product) of their union a lot of women see their children as being the center of their universe.
It's not uncommon for couples who put all of their energy into their children to lose sight of one another when they have an "empty nest". Also a lot of men have been known to cheat once they have been pushed to background status in their homes.