I have to agree with everyone else here.

You have two simple choices:

1. Accept her child and the fact that while inconsistent the child's father is a part of his life.

2. Leave.

Even IF you guys stay together and manage to work through this and get married you do not have any right to make demands regarding her child and his father. You are not the father and should not strive to be or attempt to replace him. At best you should be a friendly role model, someone for the boy to look up to. I have been in this situation personally and pushing for YOUR way will only end in disaster.

My two cents, take it or leave it.

Brandon