I think you are being unrealistic. Whatever your benchstick is for husbands who have left the family unit, it maybe idealistic.
It completely depends on the different variables of the man's capacity for guilt, duty and his concience. As for teenage years, I would say it's quite an important time for men's involvement in a child's life, I find it strange you think otherwise.
In what capacity has she made a mistake ? she lost the prize ?
Regardless of everyones situation now, the children should be front and foremost in your life and this man has not finished bringing up his children yet.
You have to accept their correspondance, it may not be the level of involvement you like but you chose him, knew the score and unfortunately that was his previous choice, this woman who bore his children.
It would be nice for you if he were to put you above all others but he is not doing so, he is trying to balance his time, aid and affections between two families. I don't think it's going to change, if he still wants to be heavily involved in his children's life he may want that until death, his grandchildren etc, I doubt this scenario will change.
That ex wife is always going to feel she has a special link to your man because of the shared children, for some couples this never changes. Unfortunate for you but you picked him and he doesn't look like he's going to change.