Communication is key. If one person in a relationship isn't okay, then the other(s) aren't either. One partner's feelings cannot be ignored. As soon as you commit yourself to the relationship, you can no longer say, "Well, that's your problem, so deal with it." Not without some hefty communication first, anyway. (Because there have been times where there are things that I (or he) just do need to deal with, but it's always "our" problem to bear. That's the cool thing about being a couple/in a relationship.)
I agree heartily with cadence- it has to be approached right. The conversation can't be accusatory, and it can't be heated. If you get flustered easily (I do), I would suggest thinking through exactly how you want to phrase things so that they are not inflammatory, and possible ways to respond to things that you already know she'll say. Bring up your feelings on the matter, define what cyber means in your mind and why it is something you're not comfortable with, and express that you really need her to communicate with you on the subject. Tell her how you feel about her (not about her cybering, but about her- gooey stuff, sincere stuff), and that you don't want to feel hurt, but you also can't just bury how you feel about this. And while it's obvious that the conversation isn't optional (from my perspective, anyway, without doing damage to the relationship), be willing to give her space, and time to think. If she doesn't want to discuss it right away, make sure she will discuss it with you, on a reasonable timetable, preferrably before something similar occurs again (if it even does occur again- she might not even be planning to do it, and that might be why she doesn't think it's important). I know, I'm getting preachy about communication, but I've watched so many couples disintegrate because they forget that they're actually on the same side! So it's something that is more valuable than gold... or, in my case, my perfume oils.Best of luck, and I hope once she realizes how important it is to you, that the situation is able to be resolved in a way that works for you both!