It's highly likely that Emma's DP is a shared fantasy of the couple, and there's a good chance MC planted that in her mind. Why? Because it's very possible that the content is in the porn MC gave to her.
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This is a common storytelling pattern for Dimajio. When a couple shares their secret fantasies through conversation or interaction, it doesn't just end there. They confirm and reinforce those fantasies mutually through sex. This is what psychology calls positive reinforcement, where their fantasies are rewarded through sex, making those fantasies a reality.
For example, MC reacts positively to Emma when Martin looks at her. Then, they have sex. This reinforced experience subconsciously makes Emma horny and her fantasy stronger every time Martin looks at her. His stories follow this pattern.