When I was in treatment, the subject of me and the wife having a child came up in a group session with me, my counselor and my wife. We didn't have any children at the time.
My wife was concerned about me relapsing after we had our first child. She really wanted children. The counselor asked her if she was okay with being a single mom if it came down to it. And she was. Thus, we had a child and are working on our second. Not pregnant yet, but trying.
Even though there were no vows said when conceiving the child, the sentiment is the same. I didn't vocalize any promises that I would be a father to the child, better for worse, or anything like that. Nor did I think that now I needed to stay sober forever (not just for today) in order to remain a good father to my son.
In addition, what if he weren't an addict? And what if after you were married, he did something horrible like commit murder or some sort of pedophillia (extreme examples, follow me on this one)? You promised, for better or worse, right? This is something everyone could potentially face, not just the future spouse of an addict/alcoholic.