My dh was adamant he didn't want a second at all. Then he'd sometimes would have had a drink at a party and say "oh maybe one day...."
Which caused me a lot of pain and heartache. If you are adamant then you have to be resolute.
I had made my peace when Ds was 5 and settled in school and I was enjoying work. Then dh started talking about getting a bigger house in a few years and maybe having another child. I said no, I'd had enough of him treating it as a pipe dream and I was going to start focusing on my job and my life.
Then matters were taken out both our hands when I fell pg with dd when Ds was 7.
He was a bit of a self absorbed dick during the pg despite wanting to go ahead. All "I didn't ask for this", "I love Ds do much, it's going to really affect him".
I ignored him, got through a difficult pg and hospitalisation had a brilliant birth.
He wouldn't be with out her now. Loves having two dc and has said to others he doesn't know why we waited 
I am not going to lie, it affected our marriage. When you love someone, and have to make joint decisions like this, one person will not be happy. If you a prepared to make this decision now and stick with it and live with her unhappiness, fine. Do so and live with the consequences whether good or bad. If its simply a case of you are not sure now, and not ready, simply tell your wife that though.
It is not your decision alone, nor hers.