I agree that training would be good!
I think for the most part people don't think in realities in general. They see the fantasy or the ideal. For love, relationships, living situations. It's all about hat they WANT, not What is. So the idea of other sexual partners is the fantasy and because they are in love with their partner it doesn't occur that they can and may also love another. I think for many people they go for adding spice to their relationship. Not necessarily *poly*.
I also think a lot of people don't do a lot of detailed picturing of possibilities and so get surprised when emotions pop up especially I'd they feel they are mentally good with the idea.