My bf once said to me that he thinks a man always has a kind of love for the mother of his children. However he did add that he wouldn''t have said that in the early days. They have been divorced over 20 years now.
It didn''t make me feel insecure, but the opposite. I think it''s good to realise that once you had something special that resulted in children. I would be more worried if he had a lasting bitterness towards her.