why does it seem that monogamy and honesty together are becoming such a rare commodity?
My answer? That people, in general, do not know themselves very well. The few that do are further limited by the ability to communicate well.
Those people that know what they want and can communicate it become or at least explore, polyamory. This might be based on my own PERSONAL bias because my friends are so fuggin awesome, but every one of the greatest communicators is poly.
The few that are great communicators and are NOT poly are honest enough to have those "If I weren't married, I'd so do you" moments. They recognize their values and chose to be monogamous but don't pretend that there are no sexual temptations or interests outside of their mono relationships.
It's really, really hard to be honest with your partner when you don't even know what you mean or want as an individual. It's just that many people default to monogamy so you see people who are all bottled up also often defaulting to monogamy. I don't think it's a "monogamy thing" itself.