When we stopped, I reminded her of the definition of a cuckold - "a man with an unfaithful wife" - and I asked her to tell me, truthfully, if there was any point in the evening when she had been unfaithful to me.
She thought for a moment, then softly said, "Yes." I asked her when that was, and she said it was when he first went inside her. All she wanted in the whole world was to open her legs as wide as she could, and to feel him slide into her body. She said it would not have mattered one bit if I was there or not, because she thought of nothing other than feeling him in her.
That, she explained, was why she had not opened her legs for me. She was saving it for him.
I told her I understood.