No one can answer the life expectancy question for you. As far as having kids, the truth is that males with CF are for the most part, infertile.
Did your fiance just find out he had CF, or has he only just now shared that information with you? If he's kept you in the dark about it before now, I would strongly encourage you both to wait with the wedding until you are sure you have what it takes to be a CF spouse.
Please understand, I'm not telling you to ditch him. My personal opinion is that a broken engagement is better than a broken and bitter marriage. You'll be in my thoughts and prayers as you make your big time life decisions together.