Since several people, myself include, have made reference to polyamory and denied this very statement, I can only assume that you have not actually paid attention to all the responses.
Because they will find out... and when they find out, if you have not told them, then that will absolutely, and without any doubt or question, cause hurt and problems.
Telling them might cause a bunch of hurt and problems.
Not telling them absolutely will.
When this happens, don't come back and expect sympathy from me.