I agree with Marie in that we as Calvinists do not believe that God is the author of sin.But, Arminians in general accuse us of that very thing. When the person you are conversing with included the sentence about "rape" he was, in my opinion, saying that you believe that God is the author of sin.
If this person will listen to your view I would start with Rom.5:12,15,18,19 and try to make him see that we believe in a Federal view of the fall of man where Adam perfectly represented us all.
God did ordain the fall in the sense that he chose to allow it,but not in the sense that he chose to coerce it.God's decree of predestination was made before the fall and in light of the fall.

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