<human>: Can you condense the text into a few key points?

We've been together 7 months. 

I can understand the appeal of "two ~~chicks~~ dudes at the same time, man". Having two hot chicks do sexy things to me would be a memorable experience to be sure! However, I see it as something one does while single where the chance of confused feelings/someone getting deeply hurt is minimal/nonexistent. She's brought it up twice over the course of our relationship and both times I've voiced being uncomfortable/hurt about it.

I want my fiancee to feel fulfilled sexually but I also do not want to feel inadequate. Is it wrong to feel inadequate about this? She says that it's not that my sexual attentions are insufficient, but that having two people focusing their sexual attention on her would be a more intense experience. I have trouble coming to grips with that. How are my attentions not insufficient if there is a more intense experience available? I don't think I could look at my fiancee giving another guy's dick attention and not be immensely hurt by the experience.

Could someone please explain to me the mentality behind the fantasy? I want to understand her perspective.
<bot>: Fiancee wants two dudes at the same time. I'm confused and hurt by the idea of it. Wat do?