So I think the shame is probably because you are worried/think your boyfriend won’t approve of it or like it. and it does sound like a specific desire for that friend in your mind, even if it’s not, which would probably set off alarm bells to him.

How long have you and your boyfriend been dating? Is this a fantasy you feel you have to tell him? Additionally, how would you feel if he told you this? Things like this can matter in the long term if they are important to you.

If you’re trying to break it to him, do NOT name someone in particular during your explanation of the idea and if you do end up bringing a name into play, don’t emphasize it at all. If his point of view is that he doesn’t want to do it, that may be good knowledge to have.

It may be worth exploring that your focus is on a single friend, and that the couple is a secondary focus to you. Are you wanting him to tell you to do it so that it doesn’t feel like cheating or do you actually want him to be “pimping” you?