I agree that you need to work on speaking up and not letting people trample you with what they want. Communication should happen beforehand, not afterward. Establish your personal boundaries and stick up for yourself! You were not really respected in this situation much at all, and I would be very angry if I were you. Why are you so willing to accept the role of daddy when you had no say in it at all? It sounds like you are seriously being taken advantage of. Don't take shit from anyone, no matter how much you love them!
I hate to say this, but . . . how can you be sure you are not being deceived? As I read your story, I wondered if your wife and S. actually planned this in order to for her to get pregnant. Sounds like she wanted a baby daddy and got one who is willing to be a donor and then step back. What about money - will S. at least have the decency to contribute financially? There are all kinds of wrong in this situation, and I feel for you, but you really need to strengthen your backbone and address how you've been treated or you will set a precedent that allows for more of the same kind of disrespect.
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein