I wouldn't call it a lie if both partners actually would make that descision out of an absolutely free and conscious choice.
And by free choice, I mean being really free to choose between several options. Which means having dealt with jealousy, fear, society issues etc and not choosing monogamy as a coping mechanism or because of idealized and overly romanticized Hollywood bullshit.

That being said, I think if people would deal with those things, monogamy would be a non-issue and only few would walk that path. Those that do absolutely have the right to do that.
But I also have a quite wide definition of polyamory...