He found out he had a child from a one night stand that happened before met someone worth sharing his future with. Maybe he didn't tell you right away because he didn't know how. Or he didn't want to lose you over something that happened before he met you. Or he felt guilty. Or this is not the way he pictured starting a family, and he had to process that. Or because his plans for the future now have to involve his child, and that changes everything for him -- and for you if you two are going to have a future together. No matter the circumstances that brought it about, he is a father. That is going to be a fundamental part of who he is for the rest of his life. If you want a future with him, you need to not just accept that but embrace it, because his daughter is going to be part of that future.