Responses
Because in the novel, the prince considered a poly relationship between them. All he needs is Louise and Simon, so all he wanted was a way for the 3 of them to be together forever, but he noticed that neither him nor Simon really desires to share Louise love.
He thinks of the idea in chapter 28 here but straight away adds that it makes him uncomfortable.
Louise doesn't like Simon that way and Ian isn't ok with sharing her. How do you imagine this would work? Nothing personal against people who are in to that but I know I wouldn't be happy if my man paraded around with another hoe on the side.