If a partner doesn't know what her/his significant other is doing, it's cheating. But people can also be hurt even if both don't have a partner. I fooled around with a guy online some time ago, when i was still single, and although i always made it clear that this will remain that way, online i mean, he still fell in love with me (i can't really imagine how something like that can happen, he never even saw a picture of me).
When i finally was able to make him realize that i am not interested in more than "fooling around online" he was terribly hurt and suffering a lot. And i was terribly sorry and don't want people to get hurt because of me having some fun.

So i keep chat to "informative" talk about BDSM or everyday topics. If possible, but that's yet another topic...