"when does something online cross the line to being unfaithful?"
This happens when you hide what your doing from the one your saying your faithful to.

There are two sides to this coin. When you explore a fantasy with another person that is not your partner, BDSM or vanillia, your removing the chance that your partner might want to also experience this with you in some degree. Participation may not be required, but observation might. The otherside of the coin is if you voice your desire to do something, and their reaction is to tell you "If you do such a thing, we are through". Then you know without a doubt your chances of being able to explore this fantasy are nil without the possiblilty of being unfaithful in some manner.

As we all know all too well, within BDSM or any other person to person dynamic that requires trust as the basis of the activity. Communication is paramount. If your not able to communicate your desires, intentions, and dreams to the one your with. Then it is time for you to evaluate your station in the relationship, and make choices.

The follow up to all that is, if your going to lie to your partner, and break a very important trust, then how is the person your trying out this fantasy with to know your being truthfull about what your telling them. Depending on the activity, this could be a crucial breakdown resulting in a less than satisfactory experience of what you had dreamed about to that point.