How you feel is how you feel, someone might not agree with it but that's just how you feel, and that goes for both of you.
If you are not sure if you ever want kids, that is one thing so obviously if she does then you don't share those long term goals. But if you're saying you don't want kids yet, we'll it's a serious thing and you do BOTH need to be ready. You haven't done a terrible thing, you were just being honest about how you felt, everyone is entitled to change their mind