I think it all depends by what "flirting" means.
I believe that in everyday life, some of us flirt -it comes naturally to some, and means absolutely nothing! In my younger days, I was like this but it did not detract from my relationship or my love of my husband.
It was simply my manner!
So, online I do not see why it is different -the question was FLIRT and nothing else so I do not see why that would affect a good relationship as it is harmless.
If it is not harmless, then I would not call it "flirting"...but something else.i.e. to me flirting is a harmless thing.
Definitely yes. At any time when you are breaking the trust of your partner, you are cheating. It has a great impact on the relationship. As Wendy said, if your partner reads what you have written and is uncomfortable with it, you've crossed the line.
Flirting,as I have always understood it to be,is an innocent bit of fun between the opposite sexes.Once it goes beyond being innocent,it also goes beyond being Flirting! Cheating is cheating,whether on the internet or in person.
One of the possibilities is that "innocent" flirting whether in the virtual world or the real can lead to other possibilities that are not so innocent. Why take the path where you don't want to end up? Prevention is better than cure, don't you think?