I want to marry my gf..?

I think you should marry her if you both feel the same way about each other and are ready to formalize your relationship. I ume that the children know the two of you are partners and understand what that means? If not, you should definitely bring them up to speed before going through this. As far as the legal consequences, nothing much will change. You may or may not be able to adopt each others children, but that doesn't change whether you are married or not. Good luck to you and congratulations on reaching this point!