What would you do if your Significan Other NEVER wanted to have a baby - but you were so in love?

My boyfriend and I have been together for 3 years. In the beginning of our relationship he asked me alot of questions about whether or not I wanted children, I was young at the time and said, "well yes of course." Several months later he told me that he had had a vasectomy after his daughter was born (she is 5 now and he was married before) but that he would consider having one more child. A year later he told me that he didn't think that another baby was something he wanted. (Did he lie to get me where he wanted me or did he really have a change of heart ~ I don't know.) By that time I was head over heels at the time and didn't stress over the issue - I figured he would change his mind or that he might not even be the man I married anyway. Here we are 3 years into a relationship and I am totally in love with this man and couldn't imagine my life without him or with anyone else. He still stands firm on the baby issue and is talking about marriage and buying a house now. I am freaked out! Yes, I love his daughter like she is mine but she obviously isn't. I am happy now but what if I am not years down the road? What if we split (although we are so in love) over this issue and I still can't have a child for some reason? Then I would have obviously made a huge mistake. I recently found out I might have some health issues with pregnancy as well. As you can see, it isn't black or white its a very very grey issue. I feel like I soon have to make a huge decision on an issue because time is running out and we are talking about the next step in our relationship. I sometimes feel like if I made him choose (which is mean and I wouldn't) that he would choose the not having a baby and losing me over having a baby and having me. Think about all the issues here with an open mind and tell me what you would do? I would love to hear if you have been in similar cirstances?