Threesome, yes or no?

So my Fiancé really wants to have a threesome, purely to watch me get sexual with another girl, he has insisted that he won't touch her at all..
I'm bisexual, so that part I'm fine with.
However, due to him cheating on me a year ago (asking MULITIPLE women for naked pictures, including a girl he'd slept with just before getting with me, and a girl I was friends with) I'm extremely paranoid and scared about the idea.
There's also the fact that I want to remain 100% faithful to him; and being sexual with another girl is unfaithful in my eyes, even if he asks me to..
Can anyone point me in the right direction?