Speaking medically, a white mucousy discharge is common from males as well as females during sex, and especially after an orgasm. Although this can be messy and sometimes inconvenient it really is quite normal and nothing to worry about.
As far as the pregnancy issue, I can't speak personally as I don't have CF but there is some medical evidence suggesting it is more difficult for SOME females with CF to get pregnant because the thicker mucous makes sperm travel very difficult. I would definetly use some sort of birth control method and/or condoms if you aren't tying to get pregnant right now.
Also, if at some point and time you are trying to get pregnant and find it difficult, there are many resources out there and you might even find information about that by searching old postings on this site. Something common I have heard women talk about is using Robiutssin (the kind without a cough supressant because you never want to supress a cough in someone with CF) because it thins out the mucous allowing the sperm to move more freely.
Best of luck,
Julie