polyexplorer, these are obviously questions from people who cannot wrap their brains around the concept of honest communication in relationships! Honesty is a cornerstone of polyamory. Personally, whether I am in a polyamorous relationship or not, I never want to be the reason someone lies to their spouse -- whom they supposedly love! I have done it in my past (when I was young and irresponsible) and I know I would feel dirty -- it would not be good for my self-esteem nor for the cheater's. I hurt people when I did that, and still feel bad about it.
Hiding and sneaking around to have sex with someone because they won't tell their partner about it is STILL lying and cheating, no matter if one is upfront about it with their own primary partner. The majority of people who truly understand what polyamory is, wouldn't tolerate that kind of bullshit.