The king gave his arm to the queen, she rose from her chair as gay as if she had been enjoying watching her husband flirt with me; but as he turned to lead her away she paused and her blue eyes looked long and hard at me, as if she were saying goodbye to a friend. (2.166)
Mary and Queen Katherine have an unusual relationship. Mary is very young as one of the queen's ladies in waiting. Perhaps Katherine sees Mary as a daughter, and that's one reason why she feels so betrayed when Mary has an affair with her husband.
"Sir, I am sorry, but I love the queen. She's a great lady and I can't betray her." (2.203)
This is an interesting reason for not sleeping with someone else's husband. Would Mary have reservations about being the king's mistress if she didn't actually like the queen?
At once [the queen] looked to me, her sharp gaze accusing me of betraying her most intimate secret. Minutely I shook my head. She looked for Anne in the dancers and saw her, with George's hand in hers. Blandly, Anne looked back. (5.72)
Anne doesn't care about betraying the queen, so she never tries to deny it. Her non-denial is pretty much an affirmation.
"I love the queen. She's a great lady and I can't betray her. I cannot take her place." (12.13)
Whoa, déjà vu. It didn't work the first time for Mary, and it doesn't work this time, either. She is still ordered to sleep with the king.
Something of the sense of betrayal must have shown in my face, because my uncle laughed shortly, kicked a log into a spurt of flame on the fire, and gestured to George to seat me on a stool at the fireside. (14.188)
Sometimes Mary is an accidental betrayer, like here, when she accidentally lets slip that Henry thinks his marriage to Katherine is cursed. With the Boleyn family around, it's a guarantee people are plotting to betray each other.
"I cannot be charged with treason, I am the Queen of England, I am England. I cannot be divorced, I am the wife of the king." (17.212)
By cheating on Katherine, Henry doesn't only betray his wife; he also betrays his country. Just take a look at how quickly things begin to decline once Henry gets his divorce. When his marriage falls apart, the whole country suffers.
"Not very loyal," he remarked.
I picked it up. "Not very polite to stop my servants and read my letters." (18.65-18.66)
Mary is a bit of a hypocrite here. Just about ten minutes ago she read the queen's mail and told her uncle all about its contents. Doesn't feel good to be on the other side, does it, Mary?
"I knew you would tell your uncle or your father, or the king. […] I knew you would betray me." (22.30)
The queen doesn't take it personally when Mary betrays her; she understands that Mary has to be loyal to her family first—even if her family is a nasty piece of work. That's why she can still treat Mary like a human, and almost like a friend.
"She's my sister," I said passionately.
"And I am your queen," she said, like ice. (29.26)
When we said the queen doesn't take betrayal seriously, we meant it only regarding personal matters. When Mary spills the beans about the queen's marriage troubles, Katherine brushes it off. But when Mary exposes a secret code from Spain, that almost amounts to treason in Katherine's mind.
There had been a trust that the king was wise and strong and that the queen was beautiful and good and that nothing could go wrong. But Anne and the Boleyn ambition had opened a great crack in that unity and now everyone could see into the void. (44.39)
The biggest betrayal is Anne forcing the royal family to betray its people. From here on out, the people will have a tough time trusting their rulers, and the power of the monarchy will decay year by year.