I really dislike the primary couple privilege thing. There have been some adjustments that we have had to make because of the mono/poly nature of my relationship, but I was very insistent on the "same rules for everyone" when it came to disclosing private information. It's not about keeping secrets, it's about what part of the relationship is private and which is not.
In my opinion there can't be any blanket rules that always apply - it's up to individual preference, boundaries and needs. It is vital that these be discussed during the initial negotiations with all the involved parties, so that expectations can be set.
And, if the shoe were on the other foot - if I were wanting to get involved with one member of a couple and were told that all details of our sex life would be shared between them, but that I wouldn't get the same from them, then I would say thank you and walk away.
"Listen, or your tongue will make you deaf." - Native American Proverb