"If it was true that a sexually repressive culture caused a predisposition, would it be racist to say it? "
If you could prove it beyond a doubt, and that it was the sole reason for such behaviour no, but until such time it remains firmly in the "all black men have big willies which they want to use on our white women" league.
You haven't even taken the firt tottering step of proving that (a) All the members of that culture are sexually repressed (nor how it effects the dominant males to the extent of making them potential rapists) and (b) How it appears not to have affected other communities with strong and distinctive sexual morays.
Nor, for that matter, how it appears to have no affect in their native country where sexual laws would be far mor stringently adherred to.
Jim Carroll