"I don't believe that signing a piece of paper makes me more committed to someone or makes it so that I wont leave someone."
I'm in total agreement over here, if we're talking emotions. But legally, you don't have a relationship with this man. Your home is NOT legally his. You do NOT have any legal right to expect financial support from him, not he from you. Both of you have legally enforceable financial obligations to your kids (in his case, to all of his kids), but not to each other. And you're both choosing to keep it that way. I certainly didn't intend to offend you and I'm glad you're not offended. But when you think about being a SAHM, those legal details loom large if the relationship should break up. All seriousness, he could just not come home one day and you'd get nothing in the way of support for months or years, if ever, and when you did it would just be whatever amount of cs the judge decides to award to 50% of his kids.
Even with as nervous as that makes me, I still think it's worth a risk and a hard transition to a even more penny-pinching lifestyle in order to be SAH with very young children. I support you in that, even more than I support you in staying with the boyfriend (and I truly DO support you in that as well). It's amazing how cheaply a family can live, provided that one of the members isn't sabotaging the budget.
Maybe Dave Ramsey would be a good idea for you guys. They run classes at churches all the time. Maybe if he heard the revelation from somebody else, he wouldn't feel threatened. Ultimately, while his feelings are important, they do not trump reality. If he FEELS threatened by your control of the $$$, and the REALITY is that said control is the only thing keeping the rent paid and the cupboards filled - then you are living with somebody that you can't succeed in life with, and choice is between keeping a man and protecting your kids from a life of insecurity and want. In other words, no choice at all.