@Ontheoutside,
I think you need to step back a bit and re-assess what you're seeing.
It seems like you're placing blame on another woman because she expects her x to contribute to the financial and emotional well being of their children. THEIR children. I know, being a step parent is hard and yes, there is a place for you. But it is not your place to blame her or think she's taking advantage of the father of her children.
And you know, she's probably right, if they go to Court, Title IV will calculate child support at a much greater rate than he's paying now. That's also why he doesn't push the situation either.
Your gripe really shouldn't even BE a gripe at all. What you're seeing is a man doing the right thing for the right people at the right time. This is probably how he'll be treating you if something like this happens, that you have children and things don't manage to work out.
If you accept these children as you say you do, then you are going to have to trust the man with whom you live with that shares custody of minor children with someone else. And, if you feel he's being taken advantage of, just know you're only seeing one side of the story.
If you really can't manage to wrap your head around this, re-think being in a relationship with someone who already has children from another relationship. It doesn't get any easier from here. Problems get messier, more complex, more expensive and more time consuming.
If this isn't what you want, be an adult and admit it and get out.