I think that it is important that the little girl be able to grow up with some image of her parents not hating each other. It doesn't matter that they were never a couple; they are TOGETHER her parents, and they owe to her at least one picture of them all together. They owe her that image emblazened onto her brain, at the foundation of her healthy upbringing. She has the rest of her life to see that her parents aren't together and will never be together.
I mean this in the most helpful and loving way possible: You are NOT her stepmother. MAYBE you will be in her life forever, but until you actually get married to her father, you are NOT her stepmother. That means that you do not get to make any decisions here. (I say that to you as a stepmother who understands the frustrations that come with the role.)
Also, I have concern for your personal health and well-being, that you would even embark on this path with a stranger. I believe that there can be an exception to anything, so I don't know that you're not it. From this side, though, I think that you should have cut your losses on that first day of meeting, before you got involved with him. So much goes into the idea of pending parenthood, especially if it's unexpected. You should have given him plenty room--unattached to you--to figure out this life change, what his role would be, how the two of them would work it out. He should have had the right to weigh his options and even decide if he wanted to create a home with this woman, after all, to care for the new life that they had created. She has every right to feel what she's feeling, knowing that YOU are part of the decision-making team for THEIR child and how HE gets to respond to the situation. Add to that that you are not his wife, and.... The circumstances under which this child was conceived are less than ideal--and she needs to take responsibility for that--but she has not had a moment alone with the co-creator to bask in their creation. It's her doing, but she's still got a right to feel how she feels about it.
He should have been "free" to decide with HER if he would participate in the pregnancy phase and the birth and the early infancy phase. Instead, he had to consider your feelings and how all of this would affect you. You should have wanted more for yourself than a bucket of drama at hello. (Seriously, he found out on the very day that you met? I probably would have seen that as a sign to keep on moving.)
If you do marry him, please wait another few years. Give him space to learn how to parent his child and co-parent with her. In the meantime, please get some counseling for yourself and with your fiance, so you can better navigate this situation moving forward.