There are many dimensions to a relationship and as much as I'd like to believe I could find someone to exist with me at all levels, some aspects of a person's life may simply not suit an SO. There's certainly nothing wrong with that and I imagine a relationship can still flourish and be fulfilling regardless.
But let's say your SO is heavily into the BDSM scene or some other fantasy that simply doesn't tickle your fancy. You've tried a couple of times, but it simply doesn't suit you. Your SO understands and doesn't bring it up again. How sweet.
Your SO still has this strong urge to live out that fantasy, though. Sure there's plenty of porn online. There are also plenty of opportunities to build a rapport with someone interested in the same sexual fantasy.
So your SO finds a little domme or sub online to have a little sexy banter with. He or she may live thousands of miles away, but your SO is communicating with them and in a sexual way--with the idea that talking to you in such a matter would cross your line of comfort.
Would you consider this cheating if it was strictly a fantasy being act out virtually with another?