That's just because Amy "raped" Tim. Yes, it was a form of rape. They'd made their pact early on. She knew about his sexuality and signed on regardless because I'm sure she loved him. He loved her regardless of his sexuality. The consummation of a marriage is written in the law & in the books, not in the heart.
Somewhere along the line she changed and could no longer sustain the verbal contract they'd made in the atmosphere of unconditional love & devotion. That's bad enough, but, forgivable, I suppose, to a man like Duncan. That she went on from there and spoke words to "strangers" is unconscionable. It's tantamount to a rape.