Why does it have to be this way?

For those with children, what do you tell them when they ask why a certain parent is never around?  How can you make them understand it isn't their fault that their father is not around and doesn't want to stay with them.  He doesn't want to watch them grow up, become adults, or help Mommy pay doctor bills, or make sure they have food on the table, a roof above their heads or items they need to stay healthy?  How can you tell a child that they can't have a particular toy, go see a certain movie at the cinema because mommy have to pay rent, buy food, gas, etc? 
Everyone tells you starting out that it is going to be hard, tough, but no one ever tells you what to say to your child when these questions come up.
Then you have the times when the child goes to visit this parent and he decides his "mistress" is more important to him and he could care less that the child is there, waiting for him to show some interest in his own flesh in blood.  All I know is I hope and pray that when the child grows up he doesn't become like his "father" sperm donor!!!!!  That would just be too much for me to digest.