It's good to explore these feelings now...not necessarily to prevent you from making a mistake (as you might fear), but simply that the sooner you understand better the complexities of your own heart the better the two of you will enjoy life together. Does your fiance handle frank talk pretty well, or would such a discussion scare him and trouble him? If not, if the two of you can talk about your fears and emotions, and what might give you pause, you can hear his point of view now and he would understand you better if in the future this issue comes up again. And of course, if you can see what his hangups are that you don't already know, you will also understand him better too.
Another thing is that you actually don't know how you will feel (and how your fiance will feel about the kids) until you do have children. The effect may very well be one that brings you all closer together. I think some things to keep in mind tho would be the lifestyle-changing effect that children will bring, e.g. greater difficulty in finding time alone for the two of you, and the stress and work involved in raising kids. Some couples deal with this very well with ease, while other couples can't handle the strain and break apart. If the two of you have such a strong relationship now and if your fiance enjoys children as much as you say, I think you wouldn't have to worry much about the latter possibility, but it is still worthwhile to think about how your relationship might change with children....and not just in the abstract sense of "will he love me less" but also in the nuts and bolts of everyday life and how routines you like might be altered...
I do hope you two end up in wedded bliss...so far he sounds like such a great catch!