Depends. If you are just jacking off to images, gifs and alike then I don't see that as an issue. It's essentially the same as porn. If you are meeting random men in chat rooms to explore fantasies you have, I don't see an issue with that either.
But if you've entered into some kind of relationship with another chatter where you regularly do sexual stuff together online, then that's emotionally cheating on your partner. Not as bad as actually cheating, but still not good.