i wouldn't.
i'd like to think my boyfriend wouldn't either.
He is the only person who turns me on, and i dont want or need to look at other guys. :)
[as for webcam chats.. i think they might count as cheating but only in certain contexts... there is such a thing as different-gender friendships, and thats totally different to online flirting.]
and i wouldnt get with a guy who had a girlfriend already. ew.