Chatting online (often in a sexual manner) with a person of the opposite sex - a person you have no intention of meeting in person or having a physical relationship with. You both enjoy the sexual nature of your conversations. Is this being unfaithful to your husband / wife?
I would say that you are being disrespectful to your spouse. It doesn't technically qualify as being unfaithful, because you're not officially cheating on your spouse or seeing anybody behind their back.
Just make sure you don't begin to tempt yourself into meeting this person. As long as you both just flirt on a friendly, non physical level, you're not technically being unfaithful. But I would think twice about the situation, would you like your spouse to be flirting with people on-line behind your back?