Fantasy

My SO keeps asking about a threesome, apparently its one of his fantasies which I'm fine with the thing is he tells me he won't touch her, he just wants me to. My fantasy tho is to watch him fuck her. I'd do what he wanted but I don't know how to inform him that I want to watch him with her. The whole thing turns me on but just thinking of watching him have sex with another girl makes me so horny and wet!! Opinions please?