I will always come second, and normally a spouse should come first.
Nope, not after you have children. No!
Look, my father and my mother are divorced. Not a messy situation like the family issues you see at your boyfriend´s life, but a clear divorce. My father remarried twice. He is not a christian so no need to discuss this, I want to make clear another point. Me and my sister always came first, no matter if we were 10 or 25. I have a loving relatinship with my stepmom, and she greets my mother regularily, talks to my father when we have troubles. It wouldn´t be possible otherwise, as we are present in his life, as my mum as my mother will always be. At least, when his child has a problem, needs money for college or simply wants to get married the
whole family should be present.
Now think of what you are asking here for. Do you really think, when your deepest wish come through and you share a life, would this lead him to a deeper connection with his child? I doubt. And you should think hard if this was moral.
Now to the infertility option, some very down to earth things.
Adoption can be, depending on the country you are living, very expensive. He is financially responsible for another child, this may be too much.
I don´t know how old you are, but I would take the infertility issue serious. Some years ago I would not even offer a sad thought on the option of being infertile or an infertile spouse, now as I deeply want a child It would be a hard burden I propably wouldnt like to accept
before a marriage.