Wow you have such a bad take on this and clearly have zero fucks given that the husband was tricked and believe the child is still his problem financially.
If a baby is adopted that is one thing, but this woman cheated and has the husband raising some other mans load.
I'm going to say this as plainly as possible:
If you could raise a child for eight years and then turn your back on them for any reason, you are not a good person. You are allowed to be angry at the mother. You are allowed to be frustrated and hurt. If you abandon this child, who views you as their father and their world, you are a complete monster
Do you feel this way about step parents who get divorced?
Depends. Did the stepparent act as a parent, or just the biological parent's spouse? I have a friend whose dad and stepmom divorced a few years ago. She entered the picture when my friend was already an older teenager, so she never really acted as a parent towards them.