I agree with what's been said.

Honesty is the key- anything less is a display of abject, utter disregard. It's easy to get caught up with the fun of playing, or flirting, or what-have-you and forget that there is another person on the other side of the screen.

So if you agree to be exclusive, or play on a limited basis, you'd better follow through. If you involve another person, you ought to really consider their feelings; otherwise, you should just go download some pr0n and entertain yourself.