Go to page 9, to the "Table 9. Victim and offender relationship, 2004" in Criminal Victimization, 2004 report which I can only presume was used as the source. You will see that while the quoted statistic does apply to females (age 12 or older) when considering the problem of rape or sexual assault… it's actually committed 100% by strangers to the victim when it comes to males from the same age group.

You linked to endnote 8 instead of 9. That report is for sexual assault in general, not to juveniles.

Endnote 9 goes to this pdf. bjs.gov/content/pub/pdf/saycrle.pdf

The table is on page 13 of 17 in Table 6. The numbers in that table are 34.2% for family members, 58.7% for acquaintances and 7% for strangers in the juvenile categories.

There's a slight bit of difference from the round numbers on the website, but they're in the ballpark. There's still an overwhelming percentage of acquaintances.

The breakdown for gender is on Table 7. The males are closer to the general statistic. The females are more likely to be abused by strangers.

The data from that report is pretty old, from the early nineties.