Ask her to flirt with other guys

During pillow talk you decide to take your shot and tell her about your fantasy.

"Honey, I have to tell you something, but I know it's weird and I don't want to freak you out." - you say while almost losing your breath

"What's the matter darling?!"

"I... I really like the thought of you being with another guy"

"What?! But I'm not with another guy! What are you talking about?!"

"No! I know that! What I meant was... I fantasize about that"

"About me cheating on you?!"

"Not really.... it wouldn't be cheating, I'd be aware of it and you'd have my approval, so it's not cheating"