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.