Just ask

Shikamaru didn't see any reason to be coy about it, Naruto was his friend, one of his best friends, he felt no real shame in his fetish nor did he think that Naruto, even if he didn't agree, would insult him or anything over it.

So Shikamaru knocked on Naruto's door later that day and was let in by Naruto and just asked.

"Hey Naruto, this is troublesome, but I have this weird thing I want to try, would you have sex with Temari in front of me."

This question caught Naruto's by surprise but he easily made a decision.