Presumably your SO knows that, on occasion, you will take a dump, even if you don't say when it happens. If you're going to a MP or SP of some kind and your SO knows about it and accepts it, by all means go ahead, regardless of whether they know about any particular incident. If you do it behind their back, don't think for one second it's anything but cheating.
For the OP or anyone else asking this question, ask yourself this: would your SO feel cheated on if she knew about it?