Since she is your fiance I would suggest whatever you say, to do it as a compliment to her:
"Oh baby, you are so hot, any black man would love to be fucking your white pussy right now."
"You would make a big black cock feel SOOO good being in your sweet white cunt."
"I'd love to hold your beautiful tits while you were being pounded by a black cock."
Whatever, make it positive and let her know you are supportive of it, and her.