Before I experienced betrayal (yes, it the was the full-on type...), I considered flirting to be harmless and fun. A way of getting an ego boost without actually doing anything wrong.
Now, however, that I've been walked through the hell of betrayal and come out the other side, it's easy for me to see that flirtations aren't quite so harmless. Why? It's kinda like opening the cookie jar to get a whiff...while promising yourself that you won't actually take a bite. (It's almost lunch time and I'm hungry – hence the cookie analogy.) We all know that, once that jar is opened, a bite – while perhaps not inevitable – is highly likely.
And that's much of the danger which what are frequently termed "emotional affairs". We dismiss them as harmless, which allows them to go on – and those engaging in them to rationalize what they're doing – for long enough that the line becomes blurrier and blurrier until it's gone altogether.
I've known of women who've been completely torn apart by a spouse's emotional affair – something I would have considered harmless. For some women, a strictly physical affair (ie. no-strings-attached sex) is easier to overcome than anything that involves emotional attachment.
The truth is ANY betrayal – indeed any allegiance – that takes a spouse's emotional and/or physical presence away from the marriage is dangerous.
In a healthy marriage, there shouldn't be secrets. If a spouse can't discuss a "friendship" with their partner, share the conversations/e-mails/etc, then chances are that relationship violates fidelity. My advice? Stop...now.