Guidance is required

The assumption behind romantic relationships is sexual exclusivity. She assumes that you must highly value the gift she has granted you, that of her being faithful to you alone. When you tell her that you want other men to fuck her, she sees it as proof of your not valuing her gift, her sacrifice to you. For most women, sex must be prompted and justified by love. When you first met, first fell in love, did you want to see another man fuck her? You do now, so what has changed? In other words, telling her that you would like some man that she feels nothing for should indulge in the gift she has given you denies that you still truly love her, still really value her. No one wants to feel undervalued.

Your task is to somehow convince her that because you love her so deeply and value her so highly, you want her to experience sexual delights that other less-confident husbands would withhold out fear or possessiveness or indifference. Ideally, your watching another man enjoy your wife's body should be, appearances to the contrary, an act of love between you and her, with the other man being no more than a living dildo. If you treat the event a live-action porn for your jaded enjoyment or as an act of humiliation on your part—"he has a bigger cock than me, so I had to let him fuck her"—she will either feel unloved or that you were never worthy of her exclusive gift to you.