The CFF and your clinic will tell you to keep away from CFers, for fear of cross-contamination. You can pass on bugs like pseudomonas, etc.
However, it is, in my opinion, a personal decision. If you absolutely fall in love with someone with CF, I think love overrules. There are CF siblings and CF cousins and stuff, and they see each other. Obviously CF siblings get within 3 feet of each other, and often. The one thing you have to be aware of when going into a relationship with a fellow CFer is that you have to be willing to live with whatever bugs they have, and that they're willing to have whatever bugs you have. Because you most likely will share them.
But, like I said. It's a personal decision. If it's a risk you're willing to take, then do it. I myself have never dated someone with CF, but I wouldn't count it out if I met someone with CF that I fell for. I do have CF friends, and I don't just mean online. I visit with some. The CFF and your clinic will also tell you not to hang out with CFers in person, even just as friends. But it's really your choice. I myself have done it, and will continue to do it. But that's a choice I've made, and I know the risks by doing it. So long as everyone involved knows the risks beforehand, and you're willing to take them, then I say go for it. But many will disagree with me.