It depends on how far a person takes the chats, zebra. If, say, couples get talking on places like AB. a lot of banter goes off, and suggestive talk - but I consider that harmless. If the chats move onto a more personal level, such as on msn, and the flirting becomes worse, or moves a step further by way of a webcam, then yes, depending on what's said or shown - I think that could be classed as cheating, particularly if someone arranges to meet up with someone else behind their partner's back.
Leggy - agreed - just pics - but I'm hardly likely to post one of someone else, am I!!!