No, girfriend, it's not like that.
I can't tell you how much it does NOT amuse or please me to read a woman making excuses to enable a man to treat another woman badly. I've lived in the country, the suburbs and the inner-cities of America. I've seen it everywhere, rich and poor and it wears me OUT.
Your husband may be a great man to you. At one point, he also cared about this other woman, even for an hour. You call her an 'unwed mother'. What is the point of that? It probably feels very vulnerable to you to be pregnant with a child whose value to his father is based on the father's relationship to YOU. Because I assure you, when your husband was creating the 2 year old, he wasn't understanding/accepting the fact that it was his job to support any children he made. If so, he'd not scoff a petty 400 dollars a month to support this child. If the mother were a sweet, kind woman who fulfilled all of your/his martyr ideals, do you think he would be happily mailing off the money? No, you're playing a very typical, common game that women with low standards play, maybe called 'Blame that other B*@% for all of our problems'. It gives you endless fodder for all KINDS of psychological games, games that provide decades of toxic stimulation.
It does not matter that the mother makes equal money to him. She is also at a phenomenal disadvantage in the workforce because she is the custodial parent of a minor child. Glass ceiling and sexual harassment issues aside, she has to deal with the kid. If the kid is up at night puking, she's going to be late/tired/absent at work the next day. She takes this hit every day, and your husband does not. He owes this child money to stay alive until she can make it for herself. If he provided her with non-toxic love and affection, that would really be a boost, but failing that, money is the lowest common denominator, and he owes it to the CHILD, who can't file in court herself yet. Because she's a KID, you dig?
If the mother is really abusing the child, and not just the unfortunate victim of a passive-aggressive, abusive BabyDaddy, then it seems obvious that a good man/father would do all he could to protect the child. A good man, a good father, would understand that parenting is an investment that lasts decades, and a bad year or two does NOT mean throwing in the towel. Even if she's a HORRID mother, the child still needs a functional roof in the meantime. His modest 100 dollars a week would not provide for all of that even, never mind:
-thrift store clothing
-thrift store books/toys
-transit to take child to playgroups, doctor's offices, whatever
I'm sorry you are uncomfortable with the fact that a man's role in reproduction is sharing sperm with a fertile female. Frankly, I'm not pleased about it either. Men are fertile every freaking day of their lives, and for that reason, I am wary of having sex with them. Nonetheless, dudes don't really get to have an 'excuse' to cry out shock over impregnating someone. Even the abstinence-only educators will tell him that he's fertile every day and that ejaculating makes babies.
Y'all probably won't really save for this child's education. You're making your own family which will drain your resources financially until you either retire/die or he decides that he wants a 'do over' from the children he made with you and presses 'reset'. What's unfortunate is that you, 'step mother', are already expecting this child to forgive you and her father for ditching her. I can only hope she'll be too strong to bother granting you an audience. It could go in either direction for her. She may be so confused about love that she lets you all in, which will likely prolong/exacerbate said confusion.
It's no exercise in entertaining drama to type this out for you, although frankly it represents poor prioritization on my part to post this on a chat board rather than publish a book about it. Your story is tired and annoying, but not entertaining. Maybe responding has been a bit cathartic, but that's allowed. You're welcome to thank me for sharing my hard-earned perspective.