Sure, every pregnancy is a risk. But you don't need to take <i>unnecessary</i> risks. Having a child without CF won't mean it'll be healthy--although, chances are, the most they'll have are some allergies or glasses--and yet, having a child <i>with</i> CF is imparting it with a virtually guaranteed laundry list of depressing health issues from early childhood on, alot of which you wouldn't want to wish on a worst enemy.
How this relates to already existing children, who aren't mere potentialities, is beyond me. No one's saying you should kill them or anything, or that their life is worthless, tapestry. Just that it'd be horribly reckless to take such gratuitous risks with a future child's life, when you could just as well scratch CF off the list of pitfalls your kids could face AND give a loving, warm home to someone who DOES exist and needs it now. I think you're just being defensive.
This whole topic is ridiculous. Come on, people! If you ask people who grew up in Soviet Russia under Stalin, (most) would still say they led worthwhile lives. But if you had a choice of having a child there, or in the US, what'd you do? If you had a choice between having a kid with cancer, and one without, which'd you pick? This <b>can't</b> be a serious debate. IMO (and I think it's a self-evident one), it's just being irresponsible to have more kids when you already know of such a strong possibility they'll be ill.
I would also suggest taking responsibility for your own risk-taking, instead of pawning it off on God. (You haven't stopped wearing seat belts, using smoke detectors or washing your hands because it's all in his hands anyway, have you?) If you have another kid with CF, you'll have no one to blame but yourselves when they start getting colonized with the scary bugs, or coughing up bloody foam. I doubt it's what God would want for you, just as he probably wouldn't want people to become heroin addicts, but you'll get to make the choice, and (hopefully not) find out first-hand.