I should add that there is no citation on this data; it also doesn't seem to appear in the book (at least, not that I could find via Google Books). A quick glance through Google does not reveal a plausible source for this. So where did he get it from? Probably not via direct observation (how would he have observed these rapes?). He must have heard it from Brazilians. Well, if that's true, then these Brazilian women must know it. And since nobody wants their daughter to get raped, this must mean that they have a very good reason for inviting these men in -- maybe the alternative is starvation. Recall that we're talking about "street children" here. I just can't imagine a woman saying, "yeah, he's going to rape my daughter, but I really love him!" But I think it's actually more likely that this is just the sort of rumor that the Catholic Church would want to spread, to combat unmarried cohabitation. It gets its memetic strength from blame-shifting/just-worldism: "If you didn't want your daughter to get raped, why did you shack (literally?) up with this guy?"

I couldn't find it either, but it is plausible assuming rape is similar to homicide in this respect. For instance,

Examining a variety of different data sets of this kind, Daly and Wilson (2008) found that the risk of a preschool-aged child being killed ranged from forty to one hundred times higher for stepchildren than for children living with two genetic parents.

Buss, David. Evolutionary psychology : the new science of the mind. Boston: Pearson Allyn & Bacon, 2012.