I don't have any ideas as to how you can come to understand this perspective - since it is not rational. However, I see variations of this all of the time - where people's actions do not match up with their stated desires. I think this stems from the fact that many people are not very introspective - they just react to feelings/situations ("I have the hots for this guy.") and then rationalize it afterwards with little insight as to how they got into the situation in the first place.
Unfortunately, in our society, cheating is common and poly is not. Therefore, we have a societal "script" for cheating whereas the concept of having open and honest relationships with people seems like entering uncharted waters for many. Crazy.
Me: poly bi female, in an "open-but-not-looking" V-plus with -
MrS: hetero polyflexible male, live-in husband (24+ yrs)
Dude: hetero poly male, live-in boyfriend (5+ yrs) and MrS's BFF
SLeW: platonic girlfriend and BFF
Lotus: "it's complicated"
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe